In their installation video, it looks like the terminals on the micro switch are labeled NC, NO and C. You want the C, for common.
I assume NC is normally closed and you would put the polarity at that electrode that you need at the frog when the switch is in the straight route posiiton. A $4 harbor freight voltmeter would verify that.
Henry
COB Potomac & Northern
Shenandoah Valley
Where does the wire from the frog attach to the Walthers machines?