You need to sign in with your Game Informer account, but after you do that just enter in your Gamertag and you will receive a friend request. Accept it, and you will get the car in-game.
EDIT: It's free to make a Game Informer account: you can do it off of their website, and you don't need to be subscribed to their magazine.
New way to get it: "Simply go into the game, access the storefront, and go into the "Buy Designs" tab. Perform a search for "GI Livery" and it'll be yours to download for free."
Source: http://gameinformer.com/b/news/archive/2009/11/05/news-update-forza-3-game-informer-livery.aspx
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