Problème résolu, la solution pour tous ceux qui ont le même problème, la voici, trouvée sur un forum anglais :
"I managed to fix this problems. Apparently it has something to do with the locale you are in, this is why Eastern Europeans faced the problem. So here goes the final solution, to all who may have problem with the DLC:
1. Change your Locale (System Settings / Console Settings / Locale) to an official LIVE country, I chose United Kingdom.
2. Then delete all your Lips save files.
3. Clear the HDD cache (Console Settings / Memory / Device Options (Y) - X, X, LB, RB, X, X).
Now if you start the game, create a new save file with the new Locale, the game should see the Marketplace proper.
Huge thanks to all who helped me with this. Thank you."
Pour la traduction, j'ai la flemme, mais bon on est en 2009 hein!!?Tout le monde parle anglais surtout nous les geeks de ce forum... 