I did this last night so it's pretty fresh in my mind. I found it pretty confusing myself, the process is really badly thought out and the badly translated English on Entware's site doesn't help.
Did you download the Registration program? I tried to download Moviepark once I'd logged in and it said my GP wasn't registered but then I downloaded the Reg program and it was fine.
I've answered this 1,000s of times and will report to Sticky n00b thing next, there are DIFFRENT programs to register at GamePark and Entware, the GamePark one is at the GamePark site and the Entware one is at the Entware site