Not sure if I understand your question, but if I'm not mistaken, all standard mp3 options are available to save/export a file in MixPad.
I assume you are familiar with the following steps, but just in case:
Under the File menu, choose 'Export Project as Audio File'.
In the Export Options dialog box, choose Output File Type: mp3
and then click the 'Settings...' button to choose MP3 Encoder settings.
Also - use the 'Output File' browse button to choose or create your destination folder (so you can find it)