In Steam, right click on Medieval 2 and go to properties, "Set launch options", and paste the following text:
Code:
--features.mod=mods/EBII
Or instead of EBII, write the name of the mod folder you want to play. For example, even though my version of Stainless Steel is 6.4, the mod folder is named 6.3. So to play SS I would change the launch option to this:
Code:
--features.mod=mods/SS6.3
As far as I know, the "Kingdoms.exe" was basically renamed "medieval2.exe" by the last steam update, so now M2TW has all of the benefits of that executable (better AI, etc.). Someone can correct me if I'm wrong.