Bought it sealed, or used? The retail versions of the game are Steam, so you need a Steam code to activate the game.
Check with your neighbour; if he bought the collector's edition and sold you the second code, maybe he gave you the wrong one (in the CE there is one on the back of the manual, and another on a separate card). Steam games are not transferable AFAIK, so if he used the only code, the disk is essentially useless second hand.
