Download and unzip to data/menu
http://files.filefront.com/Battle+Ed.../fileinfo.html
If battle editor crashes (like it did for me the first time), you need to delete map.rwm, start a new campaign, and let map.rwm regenerate.
Make sure new map.rwm is in world/maps/base (mine regenerated in /imperial_campaign).
You can take map folders from maps/custom (Grassy Plain, Sanctuary, etc. or from Kingdoms mods, like Crusades Grassy Plain) and copy to maps/battle/custom..
Then you should be able to load those in battle editor.
Then you can add some ambient scenery or change the terrain on those maps and save, ignoring the "integrity report".
Copy your new saved map folder in maps/battle/custom back to maps/custom and you can quickly play a custom battle choosing faction and units for each side on your new map.