The tagging feature in FLAC Frontend is designed to be used with Case's binary "tag.exe". You have to have this binary for tagging to work.

This binary is best at tagging files based on the file name, i.e. where you have a file named something like 01 - Beatles - Abbey Road - Come Together.wav. This is NOT useful when tagging files named according to the etree standard.

So leave the 'add tags' box unchecked and follow the good advice given above.