assuming you have sins 1.16/entrenchment 1.02 and want the maps to show in BOTH sins & entrenchment you NEED to place the galaxy's in the GalaxyForge folder which hides next to the mods folders.
here is some instructions that I have been giving out for a while now
first you will want/need a either or both winrar and 7zip ot extract files then download and install galaxy forge from the sins download page
assuming you have sins 1.16/entrenchment 1.02, 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