How Do You Make a Knob That Chooses Different Waveforms on Turn?

I’m trying to make a knob that chooses the different waveforms when I turn it, but I can’t seem to access it in controller.cpp.

There are not much information in your question, so I can only tell you how I would do it:
Create a StringListParameter, add strings representing the names of the waveforms and set the knob to use the control tag of the string list parameter. Now the knob and the controller can both use the value of the parameter.
Does this help?