You should just register it with steam as normal for any game you buy from a compatible 3rd party (i.e., games menu -> activate a product on steam -> type in AI wars unlock code). That said, if your key has been unlocked by someone else using a key crack, you're sadly out of luck - sorting out who the legit owner is would be far too time/cost intensive for Valve and Arcen to do on a regular basis, so there really is no way at the moment to resolve such a situation that I'm aware of.