mp3 cdrom

I have a '04 NB, with the Monsoon sound system, with a tape deck in the dash, and a 6-CD changer in the arm-rest. the manual says it will play MP3 CDs, but I've had no success. when I put an MP3 CD in, the display says CDROM, but after a few seconds, gives up and won't play it. I've tried a...