Instructions
Connect one end of the Ethernet cable to the network plug in the back of your Xbox 360. Connect the other end into the corresponding plug on your MacBook. Turn on your MacBook and ensure the wireless card is active. If necessary, set up the card and connect to the Internet.
Click the Apple menu. From the "System Preferences" section, choose "Sharing." Click the check box next to "Internet Sharing" to turn on the service.
Select "AirPort" in the "Share your connection from:" drop-down menu. In the window underneath the selection, check the "Ethernet" box.
Turn on your Xbox 360 and attempt to connect to Xbox Live. Depending on your network settings and the settings of the two devices, you may not need any additional configuration. If the connection fails, continue to the next step.
Open the System Settings section of the My Xbox menu and choose "Network Settings." Select "Configure Network" from the list, and then choose "Basic Settings."
Match the IP, subnet and gateway settings of your Xbox to the network settings of your MacBook. You can view your MacBooks connection information from the Network section of the System Preferences menu.
Change the last one or two digits after the last period symbol of your Xbox's IP address to a different number than the last digits used by the MacBook's IP address. The network will not function properly if two devices share identical addresses.