From the Games on Demand marketplace FAQ:
How can I access the game manuals for these titles?
The game manuals for Games on Demand titles can be viewed, downloaded and printed from Web Marketplace on Xbox.com. Simply locate the Games on Demand title in Web Marketplace, and select Download Game Manual on the product page.
Well, I've been to those pages and I don't see any link to download any manuals.
Am I doing something wrong or has this feature been removed?