raven is almost correct, that is the sins 1.15/entrenchment 1.01 method, the current is

assuming you have sins 1.19/entrenchment 1.05/dilomacy 1.0,  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+

if the map IS a galaxy forge and giving the instant win/loss screen then MAKE sure that EACH player has a home planet and is not for a particular mod that has extra home planets and stars

and if a map you want to play was released before 25th feb 2009 then you will also need the sins map updater from here
http://files.filefront.com/sins+map+updater+v201rar/;13585432;/fileinfo.html

harpo