Well, I use the shell extension. I set up an existing filename spec in its "mass set tags from filenames screen". Then I rightclick on a folder in Explorer and use the shell extension "MP3 Tag Studio/Retag assumed format" and select the desired spec.

I'm sure there's a way to do it within its UI, the trick is to figure out how to select a folder instead of selecting a group of files. Dunno exactly how to do it that way though.