I found a solution!! Alt M!!
With Waves plugins, I don't seem to be able to right click to assign a fader or knob on my mackie controller (actually Native Instruments Komplete Kontrol 49).
Assuming a user already has their controller set up and is able to midi assign within Studio One's native plugins or even third-party plugins, this is the solution for midi-mapping Waves plugins:
1. Set-up for mouse-over using the Studio One controller settings within the plugin window,
2. Move the hardware fader or knob of your choosing to pre-assign
3. Click on the plugin fader or knob you would like to control
4. Press Alt M on your keyboard.
Done.