I've already gone through Steam up to the payment plan part wondering what do I do with this. Is the key for free access to the Steam version, or am I just missing something?
See if any of the topics in the Steam section of the forum (http://knucklecracker.com/forums/index.php?board=34.0) answers your question. I'm not sure what you're asking, since you mention "free" in the same sentence as "payment plan" - which in itself is not clear, since a single payment transactions is hardly a "payment plan".
PS. I've moved this topic of yours to the Steam forum as well. So it will feel right at home among other Steam-relevant topics.
If it is through the Steam client itself, then you will not get a key - the game will go directly to your game library, and you'll be able to download and install it. If you're going through the developer's payment method, then you'll receive an email with links to download the game. You can also convert some information you receive through that email into a Steam key which you can then redeem for a Steam copy of the game, although that process I don't know much about.
If you already have the steam key (what the title suggests), go to steam, click '+ ADD A GAME...' (lower left corner) or 'Games' (in the top-left) and click 'Activate a Product on Steam...'. There you can enter your steam code.
That's if the code you got is a Steam code and not the code to unlock the game you download from here.
http://knucklecracker.com/forums/index.php?topic=15752.0