Microsoft is reportedly to blame for errors being reported by users relating to the Fallout 3 Broken Steel DLC. Xbox 360 version “The Pitt†was inoperable, and required Bethesda to take quick action. Bethesda has also reported Microsoft is aware of the flaw and is working on a fix to their Windows Live DRM Install, which is reportedly the root cause of the error.
Same thing happened to me. But I managed to install and run the game eventually with the add-on. What you need to do is:
1. Make sure you are running the latest patched up version of Fallout 3 (that's patch 1.5). Also make sure you are installing the right local version of the patch for your version (I accidentally installed the UK patch over my US version, and Live didn't work at all - I re-installed the US patch and Live was working again).
2. Log in to Games For Windows - LIVE, go to the "My Media" section, go to "Add-on" - this is where the add-on installation failed message should be displaying:
Not a cryptographic message or the cryptographic message is not formatted correctly. (Exception from HRESULT: 0×8009200D).
The safer option is to wait until M$ fixes the DRM issue and then re-download.