The megapack is just a collection of 6 expansions. Use that serial to register the 6 expansions separately. You can do this manually or the simplest way is to just log in to multiplayer and all your expansions get registered automatically. You can then redeem the Steam keys for each expansion.
Steam deals with the megapack as a bundle of 6 items so there is no way to give a key to unlock all 6, you have to use 6 keys to unlock each separately.
OK, I have generated four Steam keys for the four expansions I've previously purchased (I don't have the Megapack). How do i register the expansions on Steam? Everytime I'm in the game and try to launch the expansion, it keeps taking me to the Steam DLC and says I need to cough up $9.99 for each expansion. Am I missing something?