programmer is not responding arduino uno Mac OS X

I recently bought an arduino uno and when I plugged it into my computer, both the on LED and the L LED turned on. I am using mac os x version 10.7.5. My computer doesn't recognize the arduino as a modem and i get no notifications when I plug it in . Nothing happens when I press reset, either. When I tried to upload the blink sketch, the TX and RX lights didn't flash or anything, and I get this from avrdude in orange:

avrdude: Version 5.11, compiled on Sep  2 2011 at 18:52:52
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2009 Joerg Wunsch

         System wide configuration file is "Users/XXXX/Desktop/Arduino.app/Contents/Resources/Java/hardware/tools/avr/etc/avrdude.conf"
         User configuration file is "/Users/XXXX/.avrduderc"
         User configuration file does not exist or is not a regular file, skipping

         Using Port                    : /dev/tty.usbmodem621
         Using Programmer              : arduino
         Overriding Baud Rate          : 115200
avrdude: Send: 0 [30]   [20] 
avrdude: Send: 0 [30]   [20] 
avrdude: Send: 0 [30]   [20] 
avrdude: ser_recv(): programmer is not responding
avrdude: stk500_recv(): programmer is not responding

avrdude done.  Thank you.

I am new to this community and programming in general, so i replaced the name of my computer in the avrdude code with XXXX. Sorry if that was the wrong thing to do.
Thank you in advance for your time and help.

 /dev/tty.usbmodem621

Looks right.

Which Arduino are you using?

I am using an arduino uno r3

get no notifications when I plug it in

You won't, this is a Mac not a PC.
The error message says the arduino is not responding. Do you see the modem show up in the tools menu? If not doing anything else is a waste of time.
Have you opens up the network preferences?
What does USB prober tell you?

the modem does show up in tools. I have opened up network preferences but nothing shows up. What i meant by notifications is I wasnt told to open up network preferences as the getting started guide told me was going to happen.

This is what i get from usb prober:

Full Speed device @ 4 (0x06200000): .............................................   Communication device from unknown vendor
    Port Information:   0x101a
           Not Captive
           Attached to Root Hub
           External Device
           Connected
           Enabled
    Device Descriptor   
        Descriptor Version Number:   0x0110
        Device Class:   2   (Communication)
        Device Subclass:   0
        Device Protocol:   0
        Device MaxPacketSize:   8
        Device VendorID/ProductID:   0x2341/0x0043   (unknown vendor)
        Device Version Number:   0x0001
        Number of Configurations:   1
        Manufacturer String:   1 "Arduino (www.arduino.cc)"
        Product String:   2 (none)
        Serial Number String:   220 "55332333230351D04112"
    Configuration Descriptor (current config)   
        Length (and contents):   62
        Number of Interfaces:   2
        Configuration Value:   1
        Attributes:   0xC0 (self-powered)
        MaxPower:   100 ma
        Interface #0 - Communications-Control   
            Alternate Setting   0
            Number of Endpoints   1
            Interface Class:   2   (Communications-Control)
            Interface Subclass;   2
            Interface Protocol:   1
            Comm Class Header Functional Descriptor   
                Raw Descriptor (hex)   0000: 05 24 00 01 10 
            Comm Class Abstract Control Management Functional Descriptor   
                Raw Descriptor (hex)   0000: 04 24 02 06 
            Comm Class Union Functional Descriptor   
                Raw Descriptor (hex)   0000: 05 24 06 00 01 
            Endpoint 0x82 - Interrupt Input   
                Address:   0x82  (IN)
                Attributes:   0x03  (Interrupt no synchronization data endpoint)
                Max Packet Size:   8
                Polling Interval:   255 ms
        Interface #1 - Communications-Data/Unknown Comm Class Model   
            Alternate Setting   0
            Number of Endpoints   2
            Interface Class:   10   (Communications-Data)
            Interface Subclass;   0   (Unknown Comm Class Model)
            Interface Protocol:   0
            Endpoint 0x04 - Bulk Output   
                Address:   0x04  (OUT)
                Attributes:   0x02  (Bulk no synchronization data endpoint)
                Max Packet Size:   64
                Polling Interval:   1 ms
            Endpoint 0x83 - Bulk Input   
                Address:   0x83  (IN)
                Attributes:   0x02  (Bulk no synchronization data endpoint)
                Max Packet Size:   64
                Polling Interval:   1 ms

I'm not exactly sure what most of it means so i opened all that i could, as i was not sure what information you were looking for.

the modem does show up in tools.

So that means the Mac is seeing it. The opening up of system preferences only happens (if at all) the first time you plug it in. Ever after that there is no response from simply plugging it in.

Have you selected it in the tools menu?
Can you download sketches in the normal way?

Can you download sketches in the normal way?

not exactly sure what this means. i tried to download the blink sketch with the modem selected in tools and i got that message from avr dude.

It sounds like you might not have the bootloader in the Atmel chip.
Have you done the loop back test?

i havent done the loop back test (i just figured out what it was :blush:) i will try it out later though.
just to be clear it's these instructions here right? Loop-Back Test Instructions - Installation & Troubleshooting - Arduino Forum

i am out of jumper wires for now, but i recently ordered a 40-pack, so i should be able to do that soon. i have just standard electrical wire for now, along with wire cutters/strippers. will that work?

ndavidsf:
have just standard electrical wire for now, along with wire cutters/strippers. will that work?

Depends on what you call "standard electrical wire". As long as it fits in the holes, there there is nothing special about "jumper wires".

ok so i am trying out the loop back test now. this may seem like an incredibly stupid question but which ground do i connect reset to? sorry like i said im new to this

ndavidsf:
which ground do i connect reset to?

Doesn't matter. Ground is ground.

ok so i tried it out and when i typed something in nothing came back, but my computer did recognize the modem. what should i do now?

Did you remember to hit "Send"?

yes, i hit send. what do you think is the problem and what can i do to fix it?

You are down to:

  1. The serial device you selected is the right one.
  2. The USB to Serial chip on your Arduino is broken.

#1 tends to be unlikely, there aren't any other devices these days that enumerate as a "usbmodem".

#2 seems unlikely because if the chip enumerates, then it is generally fine. However, your loopback test suggests something is wrong.

You should try reflashing the 8u2/16u2 firmware (the usb to serial chip) or get a new board.

Just to make sure the ATmega328 wasn't the problem, before replacing the board, I would remove the ATmega328 then do loopback again.

how would I reflash the 8u2/16u2 firmware? would i need to buy anything to do so? Also i bought it from radio shack so do you think they had anything to do with it being broken? The package was opened but they claimed it wasnt a returned item but you never know.

I would just exchange it for another one.