its in 1.2 patch this is taken from readme
Battle Editor

This tool is unsupported by either Creative Assembly or Sega, it is
provided as is and there are no guarantees that it is free from bugs.

You can run the Battle Editor in two ways:
1. Run Medieval2.exe with the commandline switch:

medieval2.exe --features.editor

2. In the config file add the following option:


[features]

editor = true
it is then in options.