I am also having this problem. More detail:
The first time I tried to use the editor, it locked up my computer. After a reboot, I got an error message about the video driver having a problem. I restarted again and everything came up fine. I tried to run the editor again and the system again locked up. After rebooting, there was no error message. I've tried re-installing both GalCiv and the Expansion. Any time I try to run the editor, the system locks up and I have to hit the reboot button. Since the map editor was my main reason for purchasing the expansion, I am VERY dissappointed in the product.
I am running XP Home Edition with an ATI Radon 9000 video card. It is connected to a Sony flatpannel running at 1280x1024 at 75Hz.
If this is a known Video issue, it should be noted with the Expansion!
Red_Warlock