Most player-made maps currently show up incorrectly in the preview box because they have to piggy back off an existing skirmish map's layout and ID to work.
Many people have managed to port maps into skirmish from the campaign which would then be modded with mass points and spawn markers despite the fact that the game developer's have not released any sort of map editor or any other form of assistance. If you do not, you might seriously impair the game by accident. It should be noted that when using mods one should ALWAYS ALWAYS ALWAYS backup the original game files. Nonetheless, people have managed to find ways that successfully introduce new maps and units, many of which are from the previous Supreme Commander, into the game. Officially Supreme Commander 2 was declared early on to be uneditable by Chris Taylor.
