Sorry for the late reply.
I connected the signal and ground (not VCC) of the output cable of my engine's ingition module to Pixhawk's AUX pin.
I don't remember exactly which one. It was either 4,5 or 6. I'll check on Monday when I get to the lab.
I set the parameter BRD_PWM_COUNT.
Also take care not to set any RELAY_PIN* parameters to that pin.
Mission Planner displays the RPM count when you
1. Click the Tuning box to the bottom of the screen
2. Double click the black graph area
3. Tick the rpm1 box from the pop-up menu
If your engine spins (a bit faster than you can spin it by hand), it should display the RPM value in real time.
The documents should be a bit clearer about this procedure.