This happened to me when i had a dodgy copy (I.E Warez) of the game.
Went and bought a legit copy of the game and it worked fine.
It's only like 15 big ones now anyway isn't it?
(P.S, i just realised it looks like i'm being patronising and claiming you have a bent copy of the game, i'm not, just saying that's what happened to me when i couldn't play multiplayer.)