Use Soundcard input in the DSP. Open the Win Record Mixer (found in the Playback Mixer / Options / Adjust Levels for recording... and make sure that line-in is visible and selected[*].
Use a 1/8"mini-to-1/8"mini cable from the headphone jack (or line-out if ya got one) of your MP3 player (iPod, PDA, Portable CD, MD, etc...) connected to the line-in of your soundcard -- make sure that you don't plug it into the MIC because it is mono.
Adjust the Line-In level on the WinMixer, and the volume on the MP3 player to set the correct level. The DSP displays output levels VU style.
note: you will not get the track titles through the analog cable, so that's on you.