There should be a small "FX" tab on the very left side of the main performance screen. If so, tap it and the C1 graphical interface will slide in.
C1 can be toggled on and off by tapping the silver switch located directly above the "VOICETONE" text. A red LED at the center of the C1 graphic becomes lit when C1 is enabled. Tapping the FX tab again will slide C1 off screen (it will continue to function).