Which website is that? I'm still installing Steam atm.
At the top of the forum page you should see welcome (your name) and logout. At the left of those links you should see an account link.
Click that and then under profile you should see a link to "Redeem Key". Put the key you got from steam into that and then you're golden
I think you can claim them under bundle in the cstore in game, once you have redeemed you key.