I also have this problem.
I have read on other forums that other PS3 regions, but not US, got a patch to fix a couple things. Could the fact that we did not get a patch for the game yet be the reason why the DLC is not working?
Yes, this actually was the reason. The patch has been re-deployed so if you were unable to use the DLC before, please download the patch -- that should fix things. Sorry for the trouble everyone.