Hi,
If the MP3 format isn't available to you in the application, doesn't show up as a format when exporting, you may need to install the extension again. If so, download the extension and double click the *.install file.
If on the other hand you are trying to import MP3 files created in another application and they won't load, that could be a result of the MP3 being created in an application that makes MP3 files that are incompatible with Studio One. We know of at least one popular app doing that, a Karaoke application of some kind for Windows.
If everything seems to be installed correctly as relates to the MP3 add-on and it's simply not working, please create a support ticket and be sure to explain the issue in detail. Tell them if the MP# format doesn't appear as a choice for mixdowns, or if it's there and you can't import some MP3's you had from elsewhere, etc.