Bah je viens de trouver ta réponse dans la bungie weekly update de vendredi
A similar issue popped up on certain Xbox 360s with the Heroic Map Pack. Rest assured, assuming you downloaded the maps, they are there on your Hard Drive, their existence just hasn't cached to your Xbox 360 yet. In order to find them, back out of Matchmaking to the Halo 3 main menu and then click "Custom Games" and select Maps. You should find the Legendary Maps in your map list then.
If you don't see the maps, you can try to clear your Xbox 360's cache: From the Xbox 360 Dashboard, navigate to the System blade. Press Y on the HD symbol and then press X,X, Left Bumper, Right Bumper, X, X. If you did it correctly you will see a message saying: "Do you want to perform maintenance on your Xbox 360 storage devices?"