If I remember correctly, you see that error when the map editor can't find the install folder of Outpost 2. You can move your copy of Outpost 2 to somewhere more standard, or possibly move the map editor to the same folder as Outpost 2 and see what happens.
I think the default location was either:
C:\Sierra\Outpost2
Or:
C:\Program Files\Outpost 2 Divided Destiny