I'm pretty sure Galaxy Forge runs independently of the game itself.  I'd suggest uninstalling Galaxy Forge and trying again.

Note that downloaded maps do not go into the main Sins "Galaxy" folder--they do not go into your Program File Sins folder.  Instead, they now go into your "Galaxy Forge" folder which you can find in your Sins Application Data folder.  So, you have to find and open up your Application Data (XP) or AppData (Vista) folder. If you are on XP, the path looks something like this:

Drive:\Documents and Settings\<Your Name>\Local Settings\Application Data\Ironclad Games\Sins of a Solar Empire

You might also need to adjust your Folder Options in the Control Panel so that you can view and access hidden files and folders.

Also, note that not all of the maps that are available to you will work in the most recent versions of the game.  They need to have been updated.  These maps will work though:

http://tinyurl.com/sins3