Re: where is serial code on my dragon age origins guide?
If you're talking about the retail disc, PC version, then the base game's product code is printed on the back cover of the User's Manual at the top of the page.
The "Additional Content Packs" (al the DLC) have a different code, which is printed on a separate insert (about the size of a single page from the User's Manual.)
The base game code is 20-characters, five blocks of four. The DLC code is 16-characters, four blocks of four.
(NOTE: On the insert in my retail package it says that the DLC code expired on July 30, 2011.)
All of the digital download versions (Steam, Origin, etc.) had a separate email or other on-line communication that included the code(s).