How to Get Halo 3 Beta to Download

You need to clear the cache on your 360. Here are the steps.

1. Go to the system blade and then select memory.
2. Press Y on the HD symbol and then press X,X,Left Bumper, Right Bumper, X,X.
3. Once you do this a messsage will appear saying: Do you want to perform maintenance on your Xbox 360 storage devices?
4. Hit Yes
5. Go into Crackdown
6. Go to Download Content and Select Halo 3 Beta
   

Edit by Jester013:

And here's some photo evidence that Josh is telling the truth.