[quote name='boneless']I haven't done the multiplayer yet, but to even access the zombie mission you need to finish the game.
BTW I bought an OEM version with just the sleeve and key code from someone. Is there a way I can register this with steam so I don't have to use the disc?[/QUOTE]
You can play MP w/o the disc but for single player you'll need the disc. but, since the SP is a different application you can just crack the SP to play w/o disc and MP like I said, doesn't do a disc check. This is what I did.