Hi HeliEng,
As midi files "trigger voices" so to speak, I don't believe there is a direct way of converting a MIDI into an MP3, as the sounds you hear are not embedded into the midi file. The MIDI file just looks for the sound and plays the right notes in the right order. (Well that's my basic understanding of it anyway).
You should be able to play the MIDI and simultaneously record an MP3 with some standard recording software software.
Sorry not to be more help.
F - Wyg