With the thumb drive installed, Open iTunes, in the upper left hand corner of the screen is the word "iTunes", just to the right of it is the word "File". open the drop down menu. listed on the menu is "Add To Library". click it. a finder type window will appear. If the thumb drive is active it will be located in the left of the window, click it. any files on the drive should appear. select the folder or folders you want and click open. the music files will be added to your iTunes.
Good luck.
Good luck.