Even though you can create an account online and use special characters in your password, the game itself does not recognize it and you will get that generic error. Just use letters and numbers, and you should be able to connect to the Dragon Age Servers from within the game. Then all the DLC will be recognized.
Don't know why the same password only worked for Dragon Age and not Dragon Age II, but figured I would share my $.02 anyway.
Note: No problems with using the Amazon Downloader software. While you can use the Origin client to download and install the game - it has a terrible bug in that it may encounter an error, not complete the download, and delete all the downloaded data thus far (even trying to set the do not delete permissions on the folders doesn't resolve the problem). The Amazon software does not exhibit this problem and their servers can send you the files as fast as your Internet pipe will allow.
Finally, wait for Amazon to have the deal on the combo pack rather than each one individually - getting them for both for $10 is just too good of a deal to pass up.
