With mine i have a pair of wires that has to be connected to turn on the spindle. Sorry that I can't tell you where they are connected since it was done by ShopBot.

This pair of wires goes to a relay that is mounted inside of my control box. A very simple circuit ties the relay to an output line on the control board. (the circuit is on the shopbot site)

The beauty of this is that it responds to the controls that are built right in to the post processor. I never have to think about the spindle at all, the cut files include just what is needed to turn it on an off. As a nifty bonus I used the second set of contacts on the relay to turn my dust collector on and off.