Update:
cdrsubron7 made me think that I may have gotten a separate code for U-Boats. low and behold I was and I was smart enough to burn it onto the CD.
Unfortunately, when I entered it, I received a message that said the code has already been used

(no kidding, I used it 6 years ago).
I'm going to uninstall/re-install, and try to use the CD code for the main game prior to installing the U-Boat add-on as cdrsuborn7 suggested.