Hello,
Please be informed that MixPad cannot be used to transpose a song. In this case, I can recommend you to use Crescendo: http://www.nch.com.au/notation/index.html
The Transpose feature in Crescendo allows you to change all the notes in your piece by a number of semitones. To do this, open the Transpose dialog from the Score menu, or from the Score toolbar tab. The Transpose dialog allows you to choose the number of semitones to transpose up or down. Transposing by 12 semitones will transpose your piece up by an octave, leaving it in the same key. Transposing by -4 semitones will transpose your piece down by a major third, and so on.
Note that the range of semitones allowed by the Transpose dialog is determined by the range of note values in your piece. This is done to prevent you from accidentally transposing too far above or below the staff. If this prevents you from transposing far enough, try changing the clef first. For example, if you can't transpose your piece low enough while using the treble clef, change to the bass clef and try again.
Regards.