MP3 and MID are two totally different file formats. MP3 stores a digital recording of the sound, like a CD or tape recording. So it's an exact copy of the sound and it sounds the same every time you play it, no matter who's MP3 player you're using. MID is like a musical score - it lists what notes to play on what instruments, and the computer has to synthesise the sounds. So how it sounds depends entirely on the computer.
So, you see why you can't convert them? The computer can't easily tell which instruments are being played, what notes they're playing, and of course vocals couldn't be converted into sheet music anyway.
You *can* convert the other way - MID to MP3 - by basically recording the output of a MID file player.
I'll admit it's pretty unlikely you'd want to do that, though...