I followed the steps you suggested and the game now works, but the product key for the bonus content (the additional DLCs; Return to Ostagar, Witch Hunt, etc) is apparently invalid via Origin. It tells me that the key has already been used (I used the key one time, and that one time gave me the error message decribed above).
@oVentus; Glad you were able to make some progress. Now let's see about the DLC...
1. Check for your DLC both "in-game", (Main Menu -> Downloadable Content -> Available Content) and by right-clicking on the game in EA/Origin and selecting "View Game Details". If it's there, Follow the on-screen instructions for downloading and installing it.
2. Check your BSN "Player Entitlements" page. If you had registered your DLC successfully the first time around, it should still be there. Follow the on-screen instructions for downloading and installing it.
3. As an update to these instructions added the "DLC for Classic Games" link, (see below) which includes instructions for Steam installs.
4. If it isn't in your Origin Library, or in-game, or on the BSN, or it is but you can't get it to install, or if the link below doesn't work for you, you'll need "official support" from an EA Game Advisor. Contact them through the Help Center. They are the only ones who can help with account or code problems.
Good Luck!
[EDIT: Adding the link to EA's "DLC for Classic Games".]
