Apparently downloading the new XNA games on Xbox Live is not as easy as it would seem. Luckily however, the guys over at the Xbox Marketing Blog have detailed them for you nice and easy.
How to Download Xbox Live Community Games
Download XNA Creators Club Game Launcher (Marketplace/Game Store/All Games/XNA Creators Club). Ignore the others. Note: If you use "XNA Game Launcher" or "XNA Game Studio Connect", you'll be told you need a Creators Club membership linked to your profile.
Browse to My Games (Games/Games Library/My Games) and twist to XNA Creators Club.
Press Y to Download Games
Choose a game to download like any other game.
You can also watch a couple of movies that expalin the process: High | Low
