Gary, if the files have a .mid extension (suggest you check on PC)then when you have inserted the USB stick into Ty3 press one of the song buttons, then navigate to the USB tab and any folder you may have put them in. They should then show up.
If you've done that already unsuccessfully make sure you have copies of the songs somewhere then try reformatting the stick on Ty3, copy the files to it and try again.
John