The serial number is on the rear of your amplifier, just below the power inlet.
The serial number is on the retail box your pedal was packaged in. If you have lost that, it is also inside your pedal, on the bottom plate. Remove the plate with a screw driver just like you would if you were changing the battery.
If you are standing behind your speaker cabinet, the serial number is on the paper label that is glued on to the inside on the left side.
