usb mp3 files

[maintech]

I'm trying to load mp3 songs on my pa600. I install the usb pen drive, which shows in the window. I press Song play button, touch the folder shown in usb, but no mps files are shown. When I list the properties of the file on a computer, it finds them. Is there a process I need to follow to ensure they are in the proper format, and do I need to format the usb in the pa600. Or am I missing something else?