The rear panel EXP jack can be set to work with an expression pedal or an Aux switch. Factory default is expression pedal so you have to change it. From the User Guide:
"The rear panel Expression Pedal jack (labeled EXP) can be set to support three
independent momentary switches using Tip, Ring and Tip+Ring. In System Mode,
you can choose to use the EXP jack for either your Expression Pedal or Aux
Switches. Here’s how to select the Aux Switch:
1) Press and hold the HOTKNOB button and Right Footswitch simultaneously
for a few seconds to enter System Mode.
2) Turn the Knob until [UTILS] is displayed.
3) Momentarily press the HOTKNOB button.
4) Turn the Knob until [EXPTIP] is displayed.
5) Momentarily press the HOTKNOB button.
6) Turn the Knob to select [SWITCH]. Factory default choice is [PEDAL]
7) Press and hold the HOTKNOB button and Right Footswitch simultaneously
for a few seconds to exit System Mode. "
