As far as we know, mpdp to mp3 conversion is related to export MixPad projects (.mpdp files) as MP3 audio files. This can only be done using the original program and no 3rd party software will help you here. The export itself is done through the File ► Export Project as Audio file function.
There really is no alternative for this and you have to use the MixPad software from NCH Software, because without it there is no way how to otherwise convert mpdp to mp3.
Updated: July 27, 2016