To access the "Virtual Knob" you'll have to press [KNOB ASSIGN] and use the "SOUND [C4]" knob and assign the control change i.e. - Course Tune/ Fine Tune/ Level/ Pan/ Delay Send/ etc..
(In the photo above, SOUND C4 = CourseTune)
Next, go to the "Step Edit' screen (hold [SHIFT] press on activated step 1-16), use [C4] knob to adjust the parameters.
(In the photo above, SOUND C4 = CourseTune)