YOU have to go into your hegemon data folder

1-go to my computer,right click on it
2-go to local disck,right click on it
3-go to program files,right click on it
4-go to SEGA,right click on it
5-go to Medieval II Total War,right click on it
6-go to mods,right click on it
7-go to Stainless_Steel_6,right click on it
8-go to data,right click on it
9-go to world,right click on it
10-go to maps,right click on it
11-go to custom,right click on it

12-YOU ARE NOW IN THE FOLDER WHERE ALL MAPS ARE PUT
SO THAT THE PLAYER CAN PLAY THEM IN CUSTOM BATTLES

SO NOW YOU ARE IN THE RIGHT PLACE TO LEAVE
THE THERMOPYLAE FOLDER WITH THE OTHER MAP FOLDERS
SO THAT YOU WILL BE ABLE TO PLAY IT IN CUSTOM BATTLE