Users on the Bethesda forums are reporting an error message when installing the $10 additional content. After the files are downloaded through the Windows Live Marketplace, the installation stops and displays the message “Not a cryptographic message or the cryptographic message is not formatted correctly. (Exception from HRESULT: 0x8009200D).” There are also reports of some file-size discrepancies which could be linked to this problem. Bethesda says it is aware of the issue and is looking into it.
Since there are apparently no problems with the Xbox 360 version, some are suggesting that the blame probably lies on the GFW authentication side. Even so, they really should test these things more thoroughly next time.