You need the msbvm50.dll file. You can download the setup file that will install it in the correct folder from microsoft here:
I tested on Vista to confirm this process. I'll test on Windows 7 if you have any issues getting it to work.
NOTE: I wasn't able to get it working perfectly. It has a funny way of looking for files and if you didn't use the default directory when installing JK, then those functions wouldn't have worked anyway. Really, you're probably better off just extracting the textures of the model you want to modify, create new textures, then rename those textures to match the model's. Then create a gob file with them with would run like an add-on that you could turn on and off.
Interesting program. Too bad the development didn't go very far. I didn't even know it existed!