First make sure you have the latest version of it, which if I am correct is 1.0.0.7

1.0.0.7 can be downloaded from the main thread over at TWC, just click the link in the bottom of the original post.

There is also a post talking about edit old saves from 1.2 you'll need to do a few things with the mod, the author explained it in this post.

Quote Originally Posted by ShellShock
I have just uploaded 1.0.0.7 (see the first post in this thread), which supports 1.00, 1.01, 1.02, 1.03 and Kingdoms.

It also now uses the version information in the medieval2.exe file to determine the version of the save file (I have not been able to locate any version information in the save files themselves). However using medieval2.exe this does not work for 1.2, which CA incorrectly versioned as 1.1.0.0. To override the version in MedManager, use the File, Find M2TW menu command. This will be necessary if you are trying to edit a 1.2 save file, but should not be necessary for other versions.
If that is not the problem you're having, and downloading the newest version didn't help then I am not sure what to tell you. Indications from that thread make it appear as if the author has abandoned the mod.