I've recently bought the game from steam and i can play single player just fine, but when i go to the multiplayer part it asks me for a cd-key which i haven't been provided. Where can i get it?

I also am having the same problem and would love some help with this. I really want to play it.

Steam should have sent you a CD Key along with your activation key. Check your game account at Steam - it should be listed there

Steam resolved this issue yesterday, and now it displays your key when you right click the game.

is that right click on the game icon? after it is d/l'd to your system?

As soon as you buy the game from steam you can go to your game list and right click the game there. It should have an option to show you your CD-key

Awesome information on the CD Key for Steam!