Cannot upload application to a UNO-board from Mac Pro

I have a UNO board and a Mac with OS X version 10.5.8 the does not work together. I know that the board works OK, since I have tested it with a Windows 7 computer. I simply cannot get it to work my Mac, hate to buy a new computer (with Windows 7) just get started on my project.

I get the following error from Arduino IDE when trying to upload the "Blink" application:
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 "/Applications/arduino_1.0.4/Arduino.app/Contents/Resources/Java/hardware/tools/avr/etc/avrdude.conf"
User configuration file is "/Users/ornstein/.avrduderc"
User configuration file does not exist or is not a regular file, skipping

Using Port : /dev/tty.usbmodem1a21
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

I have read the instructions thoroughly and done everything they tell me to do. Please help me!!!!

Bad USB cable or port?
Have you tried the loopback test (stickied) to see if round-trip communications is possible.
Do the Receive and Transmit lights on the Arduino blink when you try the upload?

I used the same cable when uploading to the same board from The Windows computer. So this leaves me with a possible faulty port? Have ruled out this possibilty since I can use the port for other puposes like connecting a RaspberryPi. I can however not see the leds blink when trying to uploading.
Will try the loopback test.