It is a little tricky. You should be able to find a map file in the GalCiv2 folder, and you should enter Data, and then CustomMaps folder. Aka (This is the default path for installation) C:\Program Files\Stardock\TotalGaming\GalCiv2\Data\CustomMaps
I'd like to point out that there are map editors available. The one I made is available at the Galactic Core Website. The one up there is a little limited. I've finished an update that has random map generation, but I haven't had the time to put together the installer and submit it to the galactic core. I should be able to send it in on thursday. After that, it is just a question of how quickly things get updated.