With MP4 players, MP3 files need to be converted first before they can be added. This is the way it works with my iPod. iTunes automatically does the converting for me when it adds the songs to the library and the song files on the computer stay the same, they just change in iTunes. I'm guessing you don't have an iPod, otherwise you would have mentioned it, so I'm not too sure how to manually go about converting and MP3 file to MP4. There should be some sort of little program you can download which can do it for you, but I'm not one to tell you what sort of program that would be. Try searching for file converters, but be careful which programs you choose and where you download them from.