Go to
www.atg.com, click on the small "developer" link at the bottom right-hand corner. After you log-in (I am not sure of you can register if you're not an actual customer/client), you will see a menu on the left-hand side. Go to "Product Resources" -> "Product Manuals". Select your ATG version, and there you have it.
Their site/searh is soooo slow online.
You should have a local copy on your box. Contact your marketing/technical contact from ATG and they should provide you with the documentation.
Good luck