Below is a work in progress attempt to explain how to control the synth capabilities on the Nord Stage 2 keyboard. These instructions may also be relevant to other keyboards that have a similar MIDI implementation.
For ease of reference, here is the MIDI controller list for the Nord Stage 2 (pages 52 and 53 from the manual above): midi_controller_list_nord_stage_2_.pdf
The full user manual for the Nord Stage 2 is available here.
You need to hook up your midi cable from the Motion Synth to the Stage midi in (this is the 5 pin DIN midi input).
Set the master midi channel to the Hardware port (not USB) and the midi channel the Motion Synth is sending on (default is 1 for both notes and all MIDI CC modules). If you want the Motion Synth to only control piano you can set the piano midi channel to the Motion Synth midi channel instead of the master channel.
A detailed video describing how to setup your Nord to receive MIDI input is available here.
Once you have a cabled connection from your Motion Synth to your Nord Stage 2, you should be able to motion-control any of the parameters described in the MIDI controller list above via modules that you add and configure in the Motion Tab (Main Menu) of the Motion Synth App.