The diagram below shows how to connect a proximity sensor such as the SN04-N to the Buildbotics CNC controller.

Because the Buildbotics controller only supplies 3.3v out on the I/O breakout connector, you must wire the proximity sensors directly to the power supply.

The diode protects the Buildbotics I/O from high voltages.  It can be pretty much any general purpose diode.  The 1N4001 is a good choice.