The correct way to register the code after receiving it is:

1. Go to Account Status
2. Click on add Windows or PlayStation Service which will be on the top left hand and NOT ON "Add new service Account" (if you do that you create new account!!!!!)
3. There is a chance you wont have those 2 buttons on the top left, that means you have a small square button on the bottom left side with a + and something that looks like a ball of fire. Press that.
4. Add the code.
5. Enjoy 33 free days