So thought I'd try multilplayer since being doing the campaign.
Try to join a game and comes up 'Serial Code is currently in use'.

Obviously I've not given the key out and I bought the game from steam.

What do I do now and how does someone get hold of my key?

http://www.gamesas.com/forums/viewtopic.php?f=40&t=12386

Try searching before making a new post.

Thanks, I'll try to open my eyes a bit more when searching!