Recruit a friend help

I want to get Twintania and Ive bought two extra copies of the game. I have my recruit a friend code but I have no idea where to enter it. Ive made an account but have not yet "registered" my copy of the game. When I go to register the game it does not show any place to enter the recruit a friend code. Where does one enter this info?? Do I register the game's registration code first?
Im being cautious because I dont want to waste $20 on a game and mess up.
Some step by step info from SE would be nice on the recruit-a-friend page.