I believe that the file is the MixPad project file - you will need to export the audio. (apologies if you are already past this step!)
Under the File menu : Export Project as Audio File. Then a dialog box comes up where you can set the file format and location. Mixpad will then create an audio file.
A couple of tips - When working with lossless (.wav on a pc) files, I always export in the same format, and use a conversion program (like Switch) to create mp3s.
- Pay attention to your destination folder - click the Browse button in the export box to choose/create a destination. (I recommend saving your project to the same.)