REALLY simple -
get a USB thumb drive with a gig on it or so (I use a Western Digital Passport.) Put the movie on there. carry the drive over to your xbox, plug it into the USB port on the front (there's a little door with two USB ports behind it)
bang. watch your movie.
Still simple -
get the program connect360 (
http://www.nullriver.com/products/connect360) it's great. works well. streams from mac to xbox.