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.
Good luck!