I'm just going to make this into simple steps, but first I will inform you that I am merely sharing my method of modifying the kingdoms expansion and I cannot guarantee that it will work for you. See below for step to step of running a modified kingdoms.
1. Unpack the files (found in sega/medieval II total war/tools/unpacker/)
2. Choose the files that you will be modifying, delete the text folder and descr_new_geography.txt & descr_new_geography.db file.
3. Find the .exe file of the specific campaign you want to play; Britannia Campaign.exe Teutonic Campaign.exe etc. (Usually found in the start menu or desktop.)
4. Right click and go to properties of that execution file.
5. Go to the end of the "target" line, and insert "--io.file_first"
6. Run the game and it should work.