I tried casting magic in combat against a Terrosaurus, but I was told that my shield kept me from casting magic. Sure enough, I have a shield in my inventory, but I can't see it in combat, not can I seem to use it to parry anything. Is this normal, or am I in trouble?

Almost certainly normal, though as I recall, I could see my shield when I fought. Do you also have a sword, or have you dropped it? If you only have a dagger (and shield), I think you get the dagger combat, not the sword and shield one.
You can always "Drop Shield" so you can cast while in combat, but remember to "Get Shield" afterwards, since you will be needing it later on (but can bypass it with magic).

Think I might have discovered the problem: I imported a magic user, but the game chalked it up as a fighter.

Semi-known problem, take a look here, with thanks to Baggins for the post.

Thanks :)