You can add the second day at the Vacation Center located between the Sea Lion Stadium and Voyagers Smokehouse. The second day is free if you bought a full priced one day ticket. It is $10 if you bought a discounted ticket. I believe this is where they primarily want you to do the add-on, but the Vacation Center often closes before the park closes, so you can also do it at the front of the park (but if you try to do it earlier in the day, I believe they may send you to the Vacation Center).
BTW, the vacation center also promotes time share tours, so you will see other signs there saying "ask how to get _______ for free." Fill in the blank with any number of SeaWorld experiences such as animal tours and tickets to Busch Gardens. What they actually offer is $100 toward any of those things if you complete a time share sales offer.