Ok, load the game....
Click Multiplayer...
Click "System Link"
Press the "Dashboard Button" and select "System Link"
Goto "Lobby" and join your public room, best choose the room with most users.
Press the Dashboard Button to close the Guide then click "Join Game"
Join your room....
If no rooms show, be sure your xbox's ip and its ports for link are opened on your router (don't ask if you have this problem, use google for your routers troubleshooting steps)