If you look at the schematic, you'll see that "Vc" is filtered (100uf) and protected (1N5817) +9V. So, for all intents and purposes, it is just a convenient source for 9V, rather than getting it from the DC jack. Connect a wire coming from the Vc pad to a current limiting resistor (~4k7) and connect the other leg of the resistor to the LED anode (long pin). Connect the cathode (short pin) to the 3PDT where it will be ground when the circuit is active. If you use the Madbean switch wiring, that would be the middle/middle lug of the switch. You have a active (on), bypass (off) indicator LED with this set up.