(I'm assuming that since you say you've already downloaded the game, that means you were in the Early Access, so your SE account is already tied to your PS3, and now you're just trying to add the full registration code. Is that right? If not, I'm not sure about the following.)

The Playstation Store always goes to the download page after entering a code, but if you already have the game software on your PS3, you can ignore that, exit the Store and just start the game. When you next start the game after entering the code, the game should prompt you as to whether you want that code to be applied to your existing service account or create a new one. (The description of your existing account includes nice recognizable things like your characters' names.)