So, the RegEx portion is just for looking up information in TVDB. The point is so that the show information can be detected, then MCEBuddy can either use a pre-filled variable path (when you hover over "rename and sort . . . " you can see what this is), or you can write a rename rule yourself with the option below that.
I suspect that it might not be matching correctly with the online media information, but you would need to check the logs for that specific file to see. It should appear with the “log” button in the upper right of the app.