Where is the Xbox 360 backwards compatibility update?

I'm trying to get my disc copy of Halo 2 to work on my 360, and every time I try to play it, it comes up with an error message that says "The game could not start. Please download the game again". I'm pretty sure that the problem lies in me not having installed the compatibility update, but I can't find it in the store. Anyone have any ideas how to find it?

@apparatus_unearth: Does your 360 have a hard drive in it? Compatibility comes as an update and part of the dashboard. If you do have a hard drive, then I suggest wiping it from the xbox settings and starting over. See if that helps.