Check the mixdown settings. If you aren't working with Studio One 3 Professional consider upgrading. By default StudioOne Artist doesn't support MP3.
1) Upgrade to Studio One 3 Professional which supports MP3 import and export by default.
2) Purchase the MP3 Converter for Studio One Artist.
Or use a tool like Audacity with LAME mp3 codec to convert the exported wav-file from Studio One to mp3 (pretty cumbersome). Consider the upgrade to Studio One Professional. You'll get a lot of more cool stuff with it.