OK first step you need to get signed in on the dashboard, If you can't sign in on the dashboard you need to restart your router (Just unplug it for 3 seconds and plug back in).
Second. When signed in on the dashboard start the black ops game.
third while the game is starting hit the box guide button on your controller and there is a step in the game loading when it disconnects you, after it does this wait 3 seconds and hit connect to Xbox live, you should sign in.
after this is done back out of the menu and go to the black ops menu with the TV. pull up the guide with the guide button and you should NOT be signed in <-- I wasn't signed in on this menu.
now what you want to do is pull up the guide button and see you're not signed in the press connect to Xbox live and it should sign in.
next you will come to the multiplayer menu, you should be signed in on this menu!!!! before selecting anything on this menu check that you are still signed in. You should still be signed in.
Next press Xbox LIVE on this black ops menu it will come up with a conecting screen and you should be put in a player match lobby on black ops and are ready to play. HOPE I HELPED EVERYONE.