assuming you have sins 1.191/entrenchment 1.051/diplomacy 1.011,  the galaxy forge maps NEED to be placed here for xp
C:\Documents and Settings\'user name'\Local Settings\Application Data\Ironclad Games\Sins of a Solar Empire\GalaxyForge

or here for vista/windows 7
C:\Users\'user name'\AppData\Local\Ironclad Games\Sins of a Solar Empire\GalaxyForge.

for sins 1.15/entrenchment 1.01 and earlier, the correct location IS the program files \galaxy folder, 
and you also NEED to edit the entrenchment's galaxy.manifest to include the maps and also the total count of maps

NOTE the first two folders after the 'user name' are hidden , 
so to see them you must first click on 'tools' then 'folder options', then 'view', 
then click on 'show hidden files& folders' then click ok, 
then you will be able to browse your want to the folder. I suggest when you find the folder 
that you create a shortcut to it on the desktop

NOTE 2 the 'user name' in the above paths is your WINDOWS login user name, eg Administrator

also the galaxy forge maps in the local apps galaxyforge folder only show up in the galaxy forge tab 
of map types in sins 1.16+/entrenchment 1.02+

As writen by harpo99999 ( http://forums.sinsofasolarempire.com/174739/page/30/#replies )