mloaf: Thanks guys, that was it! Out of curiosity, do you know why the keys there were different from the keys I used to redeem the game?
As an addendum to what others have pointed out, there may be another reason (I'm not sure how the multiplayer works for M&B): in NWN, for example, multiplayer requires a uniques key, but the single player protion also requires a key in order to play. Due to the way the game was coded, GOG provides people with a generic key that works in single player, but multiplayer requires you to go to your account and request unique keys. M&B may be a similar situation.
Regardless, I'm glad you got it working. :)