Just run a wire from either lug 2 or 3 on your bass pot (which are connected) to lug 5 on the switch and you're good to go. If you're in doubt and can test this outside the box, use alligator clips to check the connection before soldering it up. Good luck.
BTW - I built the Barber Gain Changer and it is GREAT!! I think you can't go wrong with any of the Barber designs.