Installing a 9vdc wall adaptor to Boss AB-2 2way Selector

[stimpybird]

I'm tired of this switcher running out of power as i leave the instrument cables connected on my pedal board. I want to install a side jack to hook up to an I-Spot  wall adapter. My problem is reducing the 9vdc to 3vdc.  Not sure of the Milli-amp rating.  What in line resistor should i use?  Please keep the answer simple if possible as I'm not a wiring wizard. Thanks