This took me quite a while to figure out as well. When you have the Basic Looper program loaded, the Looper is in the A slot. Press PERFORM. Since the middle of the three switches is mapped to the A slot, that’s the one you switch to 1-Button Looper. To do that, press the LED-backlit button just above the button to cycle through functions. Eventually you will get to 1-Button Looper.
