It sounds like to me that you are trying to play the MP3 from the computer and you want to listen to it through the DM-10, right?
The DM-10 should have an 'aux-in' that you can plug in a CD player, MP3 Player or computer - using a stereo cable that fits. Hellfire has one, and can provide you the tech details - or download the manual from a link that can be found elsewhere on this site.
basically, instead of plugging in your headphones or stereo speakers to your laptop, get a stereo cable and run it from the laptop speakers jack to the DM-10 Aux-in.
Hope that helps.