Disklavier

I have a U1 disklavier mk III with a floppy disk drive. I am finding all sort of applications for it. I have now found some midi files for piano trios. I have downloaded them hoping that the disklavier can play the piano part, leaving myself to play the cello part and my daughter to play the violin part. 

However whilst listening to midi file through headphones and therefore through the synthesiser, I can hear all 3 parts, with the violin being played by the piano, if switch to playback through the piano I can only get the violin part played.

Can anyone help me with how I can choose through just the disklavier menu system to have the piano part played back through the piano? Or do I have to edit the file using a software package?

I now see that I will have to possibly covert the file to a different type before I can even start to re-assign he tracks to different instruments and control the overall volume. Can anyone give me a blow by blow checklist please?