How do I determine what the disc region is?
I figured it out....Listed below are the region codes for PS3 games. Turns out the region code for the game needs to match the region of the Playstation Store where the DLC came from. The region code is a 4 character code and is listed on the side of the game case below the Playstation Network logo.
BCUS - sony US game
BCES - sony EU game
BLUS- 3rd party US game
BLES- 3rd party EU game
In my case, I was sold a European copy of Dungeon Siege 3 from Gamestop, which caused a problem with the DLC. I traded in the European copy for an American copy, reinstalled the game and DLC, and now it works. All of my trophies were saves, but the bad news is that none of my saved games work with the American copy. I guess I'll just have to go back and beat the game all over again.