Recheck your RC-20XL's connection, and ensure that your amp is powered "Off." If you are satisfied that your setup is fully connected, power the your amp to begin using the RC-20XL.
Hold down the RC-20XL's "Exit/Shift" button, while pressing its "Mode/Stop Mode" button. Release the "Mode/Stop" button, while keeping the "Exit/Shift" button held down. Take note of the "Normal," "Center Cancel" and "Flat Amp Simulate" "Mode" indicators -- only one light will be illuminated.
Press the "Mode/Stop Mode" button to toggle between the RC-20XL's mode. Stop when the "Center Cancel" indicator is lit, and then release the "Exit/Shift" button to select "Center Cancel" and apply fade outs to your canceled loops.