You take the card to the ticket booth at SeaWorld. Each RR card will get you up to 4 discounted admissions.
You can purchase admission on SeaWorld's website, by doing so 7 days in advance you receive a 10% discount on your tickets. However, no other discounts are allowed when purchasing online.
I signed up for Rapid Rewards at the end of March... do they send you an actual card or do you just use the one that you print at the end of registration?