When doing button installs for BBS members, I just take my digital ohmeter, set it to 0-2000 ohms, and connect it to the three leads from the knob board as shown on Brian's site. My meter supplies enough current to power the LEDs, making for a quick and easy test of the board before sticking it on the display module.

Only once did it fail to work first try. In that case, I just applied more flux (using SMT flux pen) to EVERYTHING, and then remelted each solder pad again, one at a time. When done, I re-tested, and all was well.

Cheers