So I bought Rebellion during the Steam Summer Sale, and I absolutely love it.
Awesome. 
location specified (c:\Program Files\Stardock Games\Sins of a Solar Empire\Galaxy)
That is not the specified location (actually I don't think Galaxy Forge tells you where to put them at all). That's where the default maps go, not user created ones. User created ones have their own little tab in game, rather than small, medium etc.
Anyways, to get them to appear, place them in the "My Documents" -> "My Games" -> "Ironclad Games" -> "Sins of a Solar Empire Rebellion" -> "GalaxyForge". Any maps here will be loaded into the Galaxy Forge tab in game, though note if you download any maps made from an older version of Galaxy Forge your game could crash.
I bolded that folder because that's where all other user generated content and useful things like saves, settings and replays go. You generally don't need to go the the game install directory except for getting those extra modding tools like convert data and galaxy forge.
