By default StudioOne Artist does not support MP3 file formatting.
Here is a StudioOne version comparison chart of what is included in StudioOne Artist and Professional:
http://www.presonus.com/products/studio-one/compare-versions
Upgrade Options
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If you own Studio One 2 Artist or Studio One 3 Artist:
1) Upgrade to Studio One 3 Professional which supports MP3 import and export by default.
- You can purchase an upgrade to StudioOne 3 Professional here:
https://shop.presonus.com/products/Studio-One-3/studio-one-upgradescrossgrade
2) Purchase the MP3 Converter for Studio One Artist.
-You can purchase the MP3 Convertor for Studio One Artist add-on here:
https://shop.presonus.com/products/Studio-One-3/studio-one-add-ons
Note: The MP3 add-on requires Studio One 2.6.2 or higher.
-How to install add-ons in Studio One 2:
http://support.presonus.com/entries/25607269--8-Installing-Studio-One-Add-Ons-
-How to install add-ons in Studio One 3:
http://support.presonus.com/entries/68930690--7-Studio-One-3-Installing-Studio-One-Add-Ons
3) Use a third party application to convert the MP3 files to WAV files that are supported in StudioOne Artist.
-Convert your MP3s to WAV files and you will be able to drag them into your song from the StudioOne browser.
-Export out as a WAV file and then convert them to MP3 in a third party application.
- Here is a video showing the process using iTunes:
http://support.presonus.com/entries/21502115-Converting-Mp3-To-WAV-in-iTunes