I have the same problem. I can't use maps I created in Galaxy Forge to play with Entrenchment. I copied the map to the locations people said to try but no luck.
I used a solution I found on another forum. It worked for me. Here is what the guy said:
To get galaxy files to work in Entrenchment:
You need to put the galaxy file in your
Program Files/Stardock Games/Sins of a Solar Empire/Entrenchment/Galaxy directory.
Then you have to add it to the galaxy.manifest file in the Entrenchment directory. This is a new step because Entrenchment uses a different system to keep track of where all the files are, to make it possible to stack mods. Likely vanilla Sins will eventually switch over to this system.
To add it to the manifest file, add 1 to the fileCount at the top, then add another fileName. Make sure you spell fileName properly (capital N) and make sure you have quotes around the actual name. In short, make sure you are using the exact same formatting.
Don't use the folder under user settings (on vista, it's part of the User directory, on XP, it's part of the Documents and Settings dir). It will cause instant game loss, this folder is for maps created with the in-game map creator.