See these FAQs if you find another folder: /t/964/1
Also, you need write permissions to update the mp3tag.cfg file. Perhaps the program folder is not the best place for that.
And: after you have added the source, you have to restart mp3tag.
The sources found inside Mp3tag.app only serve as a template for the Wine environment on Mac OS X. They're copied over to an app-specific directory once you've opened the application for the first time.
Usually, you'll find %APPDATA%\Mp3tag in OS X at a location similar to