I wan to convert MIDI files to MP3 on ITunes

Hi. Take a look here: http://www.wikihow.com/Change-a-Midi-File-to-a-Wav-or-MP3-File

You can also use some third party converters available on the App Store.

Do you have GarageBand for Mac installed?

• Open Garageband and create a software instrument track in the New Project window.
• Drag the midi file into the software instrument track.
• Select a suitable patch from library inspector as the instrument to play the Midi file.
• Click the Share menu and select "Export > Song to Disk". Click the MP3 button. You cannot share directy to iTunes as MP3.
• Launch iTunes and open "File > Add to library" and select the exported MP3 file.