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Create button or buttons that send note-on (or control change).
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Create a knob.
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Set its Subtype to Transpose.

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Accept the default transpose steps (affects the display min/max, number of ticks, MIDI Min/Max and default value)
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Make the button or buttons subcontrols of the Transpose knob
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That's it!
The steps are identical for Octave Transpose (except for #3, of course), and THEY MAY BE COMBINED.
How Does It Work?
The transpose knob centers around the default MIDI value. Each value above is a transpose up, and each MIDI value below is a transpose down. You can always make another transpose knob that shows how the values should look by accepting the defaults.
I Want Buttons That Say + and -
Easy enough, see Steppers: http://mididesigner.com/qa/170/create-down-pair-buttons-set-thru-program-change-other-values
I Need Discrete Buttons
Check it out here: http://mididesigner.com/qa/2868/transpose-in-midi-designer-with-buttons