I bought the original BA on Steam. Now I have just bought the Megapack from the Slitherine site. I figured that I'd be able to get Steam keys for the expansion, no worries. However, the Megapack won't even install. I get an error message that the program to be updated is "either missing or has ...

I have the Steam version too and tempted by the Easter discount, bought the Megapack, assuming that Steam Keys would be available on the 23rd. However, the Megapack won't install, with error message "The Upgrade requires a different version of the program". So you should wait before buying...