Is it exclusive only on Steam? If so, that would svck...

You have to register the game on Steam no matter where you bought it (so you have to have Steam installed).
Once you do the CK will become available to you on Steam.

This game is steamworks, wherever you bought the game for PC, you have to register to steam to play.

Edit : too slow

And don't say that your amazon copy didn't "come with steam" and you are playing without it, because that would be an obvious lie (we know you would have pirated it), and would get you banned on the spot