Are you referring to the first Xbox or the Xbox 360, the one with the Gears Of War series that is built entirely on Unreal Engine 3 in the UDK by Epic who made the UDK?
I meant the UDK doesn't run physically on the target console - they develop the game on the PC and cross-build to the console. My point was the entire UDK wouldn't need to be ported over to the Mac to get games built for the Mac.