Eh, converting MIDI to MP3 is kind of pointless. MIDI files are nothing more than "instructions" for synthesizing music. If you have a low-quality soundcard, your MIDI's will sound like crap because the built-in synthesizer engine isn't up to snuff. Conversely, if you have a high-end gaming sound card or audiophile one, then you'll have a much better synthesizer (And it'll probably support the loading of music data to more authentically reproduce the musical instruments).
WAV and MP3 files, on the other hand, are strictly recorded digital audio files. What you record is exactly what you hear on someone's else's machine, where as with an MIDI, that can change.
That all said, the easiest way to convert an MIDI to WAV (then to MP3) is to use Winamp, which has a built-in MIDI player, and enable it's "Diskwriter" plugin to write the "music" out to a file instead of your speakers. Just disable looping, and Winamp should output a file with a large size (1min in WAV == 10MB). Then you can take that WAV and use an MP3 encoder to compress it.
FYI, check out MODs. They're the "halfway" point between MP3 and MIDI. They come in a variety of formats like MOD, S3M, IT, and so on. It's old school tech, but the MOD scene is very much alive, and there's great music to be heard.
--K