No, the Imperial Edition is just an add-on.
This is a problem I haven't heard of or faced, but my gut says it has to do with Steam. My gut could be horribly wrong, of course, but here goes:
If restarting Steam didn't help, try reinstalling Steam, as that's a pretty encompassing problem solver. A light way you could do it is to go to your Steam folder and delete everything except Steam.exe and SteamApps, restart your PC, then run Steam.exe.
