You need to put your maps in the Galaxy forge folder located in the same place as the mods folder.

assuming you have sins 1.17/entrenchment 1.03,  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.

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

also the galaxy forge maps only show up in the galaxy forge tab of map types

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

harpo