hmm...
I do it with my PC where my 360 is playing off my 22 inch monitor but that is with the VGA cable...unfortunately with apple's cool and hip attitude they do not run the same VGA connection that PC does. so here is my solution it will take some research
For your mac, you have a cable that runs from the monitor to the tower, you'd need to find a cord that will connect to the xbox VGA cord then meet with your cord that runs to your monitor, I highly doubt they make something like that seeing as Microsoft is enemies with apple.
This is a VGA connection for a regular monitor/hd tv that xbox uses
you'd need something to connect your mac monitor to that.