I just beat BG1 and now Id like to play the ToSC expansion. When I choose Mission Pack in the Menu, it tells me to load final save game. And then when I do that, it just loads right before the last battle. What do I need to do? Am I missing a patch or something?

No, that's what it is supposed to do. There is a TotSC patch however.

You should have the option when you first start the game to start a NEW party, replay everything from the beginning AND go to the TotSC areas BEFORE you enter the city of Baldur's Gateg. The option you are getting allows you to backtrack out of Sarevok's lair in BG and go to the new areas.

Moved to the BG1 and TOSC forum