this^^^
we just went through this the other day to figure it out. you can also just watch the LED on the battery box. it seems like you have to hold the button a long time before it changes, but that's a good thing.
plug a cable in, press and HOLD the piezo knob...be patient... the LED will turn amber...continue to HOLD...the LED will then blink once (for mono) or twice (for stereo output). then you can release the knob.
Same procedure to switch between the 2 settings.
When you first plug in, the LED will blink once or twice to indicate the current setting.
Also, the LED blinks amber a second or two after unplugging the guitar. this shows the active circuit shutting off (and thus, not draining the batteries...)
The LED will blink amber if the batteries are getting low on power. (thanks, AJ

)
Also, I don't think that the system automatically detects a stereo cable when plugged in. I believe you still have to manually switch the system.
hope this helps.