I accidentally uninstalled system ui on my android atrix hd?

First try booting into recovery and wiping the dalvik cache (power button and Volume button down)
if that doesnt work Flash the rom, Since the previous owner installed a custom rom U shoud already have recovery on the phone it also means that the phone is rooted (custom rom = phone rooted && phon boot unlocked). Here is a link to installing the stock rom for the atrix HD (though I'm big fan of the pacman rom)
http://forum.xda-developers.com/showthread.php?t=2231249

Feel free to contact me if you get stuck on anything