Go to the Fable 3 listing on the Xbox Live Marketplace by clicking GET DEAL
Click the "Download to xbox 360" button. / Buy Game opttion
Game will be priced $19.99
Click the "use microsoft points" option (do not buy with cash)
It will be listed as Free (0 Microsoft Points)
Confirm your purchase, and turn on Xbox - It will download on your Xbox even i your region is not US
Like some people (Chanser and Lazio_UK) have noted, just search for it on the console itself and it shows up as free straightaway.
The Walking Dead is also free.
Edit - Walking Dead episode 1 is free.
-> 2 free games (to keep) every month from July onwards for Gold members. Starting with Halo 3 & Assassin's Creed 2 in July.
And no more free games after 31/12/13