Midi is instrument/note based, and so different than MP3 which is compressed wave form data. A good way to make this comparison is that midi=vector and mp3=bitmap. It's very hard to go from bitmap to vector, but the other way around is perfect.
From SoundCoverter's page: "Conversion from sample based formats (mp3, wav, etc) to note based formats (midi, ring tones, etc) is not supported."
There are some converters out there that will do it, but the results really stink. The best way to create a midi file is with a midi instrument program and sheet music.
What do you need a mid for?