Can't Download a World from the Sims 3 Store

Hi, I'd like to download a world from the Sims 3 store, but I unfortunately can't register my game through the store since it won't recognize my Steam code. I have entered it correctly three times, but still no luck. I have not used the serial code before on my account, bought it through Origin or any of the expansion packs. It was all through Steam. Is there a way to get my serial code registered, or do I really have to buy something on the store before it'll let me download a world to my game?