These are the instruction I use to connect my macbook to my xbox360 and use live:
Use ethernet from xbox360 to macbook.
Connect xbox live from macbook
Mac OS X
1) System Preferences-->Network-->Built-in Ethernet-->TCP/IP
2) Set Configure IPv4: to Manually
- Set IP Address: to 10.0.0.1
- Set Subnet Mask: to 255.255.255.0
3) System Preferences-->Sharing-->Internet
4) Set Share your connection from: to Airport
- Set To computers using: to Built-in Ethernet
- Click Start and than Start
XBOX 360
1) While in the Dashboard, navigate to the System blade
2) Network Settings-->Edit Settings-->IP Settings-->Manual
3) Set IP Address to 10.0.0.2 ( The first three segments i.e. 10.0.0 will need to match what you set in step #2 of the Mac OS X section )
- Set Subnet Mask to 255.255.255.0
- Set Gateway to 10.0.0.1 ( What you set in step #2 under the Mac OS X section )
- Click Done
4) Go to DNS Settings
*** NOTE: I set the primary to my router and the secondary to my ISP's primary. You can find your router's IP by going to your MacBook and to System Preferences-->Network-->Airport-->TCP/IP
*** NOTE: You can usually find your ISP's DNS servers in your routers status page, see the manual that came with your router for more information