Instructions
Click the "Apple" icon at the top of your computer's window. Click on the "System Preferences" icon, then click on the "Sharing" button.
Click on the "Internet Sharing" button and then choose your Airport Extreme network from the menu that appears.
Click on the "Airport" from the "Share your connection from" menu on the screen. Click on the "Ethernet" option from the "To Computers Using" option.
Plug an Ethernet cable into the Ethernet port of your Apple computer. Plug the other end of the cable into the Ethernet port of your Xbox 360. This is located on the back of the console.