Well first I was gonna say "Surely all codes are applied through SE Account Management", but then I re-read the OP.
Looks like you have to do it through a PS3... Seems weird though, might be an oversight..? Of course, you could register a SEN/PSN ID for free and go round to a friend's with your PS3 CE and link it/apply codes there. It would mean that in the future you'd be able to play on Playstation (hey, we might be playing this game for 10+ years; you might get it on PS4 or PS5 lol).