I use Synplant 2 on a Mac and would like to use the standalone app, but it doesn't receive MIDI and there is no way to set up a MIDI device. There are also no instructions in the user guide regarding this. Did I miss something?

I think you might be right that it's not mentioned in the manual.

In the Synplant app, you access the MIDI settings via the macOS menu bar at the top Synplant > Device Settings...
You can also access it via the keyboard shortcut ⌘ ,

Also worth noting that you can use the computer QWERTY keys to play the Synplant app. They're really handy! I discovered some of these just poking around, so perhaps I'm missing some?

• Q and A rows — Notes
• Z/X — Down/Up octave
  
  • Use Shift to Transpose single steps
• C/V — Lower/Raise Vel (by 10)
  
  • Use Shift to adjust single steps
• Up/Down arrows — Mod Wheel
  
  • Use Shift for slower movement
• Left/Right arrows — Pitch Bend
  
  • Use Shift for slower movement

Thanks a lot Joey, that info helped