I installed ubuntu 12.04 on my K55A about a week ago from a usb stick. Go to the ubuntu download page, decide which one you want, download it, burn it or make a bootable usb then boot your device from cd or usb. Along the way you'll get the option to try it without installing or to do an install, then you'll be on your way.

Now, I'm not on a zenbook but all of my bells and whistles work f-keys/lights. Graphics are good and battery life is better than with winblows.