I'm having problem modifying tags of files on Android phones.
I'm using an Android 6.0 ASUS Zenfone ZB500KL, my PC uses Win7 64bit Professional Edition.
After starting mp3tag, and clicking on File>>Change directory..., and choosing my device on the side panel, I'm not able to choose a folder.
I think you are 100% right as in his original screenshot you don't see a Drive Letter (just a device name) and in that case then it's no surprise that Mp3tag can't read the files. He just has to copy the files to his C, D etc drive and then read them with Mp3tag and copy them back after editing them.
On your computer system there should exist sort of a computer management console resp. disk management console, where it should be possible to assign a permanent drive letter to the drive, while in mass storage mode.