Here's how to fix the problem of getting a custom map made in galaxy forge to work in Sins; both to see the map in the first place and then to be playable (rather than instant defeat).

Firstly : Make sure Sins is installed on C: Drive (as opposed to in Program Files)

2: Once you have made the map in Forge, open it up in notepad.
3: The heading should read :
TXT
versionNumber 1
isBrowsable TRUE
browsePictureName ""
browseName ""
browseDescription "TODO - Add Description Here."
isFirstCapitalShipIsFlagship TRUE
planetArtifactDensity 0.150000
planetBonusDensity 0.400000

4: Change this to :
TXT
versionNumber 0
isBrowsable TRUE
browsePictureName "ScenarioPicture-UserGenerated"
browseDescription "IDS_CUSTOMGALAXY_DESCRIPTION_USERGENERATED"
isFirstCapitalShipIsFlagship TRUE
planetArtifactDensity 0.150000
planetBonusDensity 0.400000

5: Make sure that this file is saved in 'C:\Stardock\Sins of a Solar Empire\Galaxy' and not 'Users/Name/appdata'

Thats it! It works.  Your map will come up in Smalll, Medium etc Scenarios, and should work fine. Hope this is some help to other people who have spent ages on a map in Forge, only to not be able to play it!
Cheers,  Alex