Unfortunately I'm not too familiar with Arch Linux. On Mint (Debian derivative) It seems to just work. (Or maybe I made changes so long ago that I have forgotten.)
When I plug an Arduino Uno in to a USB port and go to Tools -> Port it shows the Arduino as "/dev/tty/ACM0 (Arduino Uno)" which makes it pretty easy to select the correct port.
What do you see for ports? (Oh, greyed out.)
Be aware that when you make changes to /etc/group you need to log out and log back in for the changes to be effective. If Arch uses udev you might need changes there as well. (I looked and don't think I have any customization on my system.)
One thing to try is booting a live DVD of some distro like Ubuntu, Debian or Mint. The Arduino IDE seems to work fine out of the box on those.
Just wrestled with this too. I am brand new to Arduino. I had this error using the micro-usb port on the Arduino. I moved to connecting the other little pcb that has a full size usb with cable....and connecting via this I got an option of a different serial port (usb) and could then upload to the Arduino.
Ubuntu Studio - Ubuntu 14.04.1 LTS 64bit
If you're running the IDE with sudo and still getting the error, selecting your board in the menu under Tools -> Port might help. That's what fixed it for me.
Remember that Arch Linux often assigns strange names to devices. A general tactic for solving the "what's that device called here?" problem is to look at the output when you run dmesg | tail in a terminal shortly after plugging in a new device. When I run that command after plugging in my Arduino Uno, I see cdc_acm 3-2:1.0: ttyACM0: USB ACM device in the output, which tells me that a recently connected device is registered as ttyACM0 rather than as COM1.
edunham:
Remember that Arch Linux often assigns strange names to devices. A general tactic for solving the "what's that device called here?" problem is to look at the output when you run dmesg | tail in a terminal shortly after plugging in a new device. When I run that command after plugging in my Arduino Uno, I see cdc_acm 3-2:1.0: ttyACM0: USB ACM device in the output, which tells me that a recently connected device is registered as ttyACM0 rather than as COM1.
I think you will find that COM1 is for brain dead systems based originally on DOS, then Windows.
All Unix systems (and that includes Mac) put then under /dev/tty* where * can change. The driver for the Uno R3 being different to standard USB to serial comes up as an ACM device.
I run the IDE under Arch as a normal non root user, having downloaded the package build script from AUR and following the instructions like add the user to groups uucp and lock.