Steam Activation Code?

[PotatoCod]

Ok, so the game is available on steam but I bought it at a store and, it being a third party game, putting it on steam is not so simple. What I want to know is, how can I add it to steam so that it is a steam copy (i.e. I can have it on other computers with steam).

Right now I have it on my boyfriends computer but I want to be able to put it onto my laptop so I can play it at home (I also want to watch my little sister play it ).

It seems to require a different activation code from the one I was given with purchase, specifically a steam activation code.

Does anyone know where I can get this? From what I've read, it seems I need to get the activation code directly from the company, but that has just led me here.