You should be able to set your Xbox to use an external MAC address in the network settings. After plugging in the network cable at either end you should set this to your Mac's network card's MAC address. To get that open up your System Preferences and choose 'Network', choose 'Ethernet' from the list of network options on the left and then click the 'Advanced' button on the bottom right. That should give you a dialog with a horizontal tab list, click the far right one called 'Ethernet' and your MAC address is the first thing listed.
You also need to make sure 'Internet Sharing' is enabled in the Sharing preferences. You need to pick a network option to share to and from. So in this case you're sharing your Airport internet connection with your wired network card (and anything connected via it).
That works for me anyway so if that doesn't help then perhaps try this: http://macapper.com/2009/01/29/tutorial-mac-internet-sharing-with-xbox-360/