You'll have to subscribe(and then un-subscribe if you're feeling cheeky) through this link with your G2G email.  You should also have your Steam linked already or, if not, link your Steam and that should fufill the requirement.  If you already had your Steam linked from before the offer it doesn't count(I don't think).  After confirming the e-mail from your e-mail account, start the game and check under content and click the box.