I am using a Spektrum Remote Receiver (serial) and a DX7 transmitter with my Pixhawk.
In the MP Full Parameter List I have set up the following RCMAP
RCMAP_PITCH channel 3 (not used)
RCMAP_ROLL channel 1 (aileron) steering
RCMAP_ THRO channel 2 (elevator) throttle
RCMAP_YAW channel 4 (not used)
RCMAP channels 1 and 2 are used for steering (aileron joystick) and throttle (elevator joystick) respectively. Your steering servo will be hooked to the Pixhawk servo output channel 1 and the throttle to channel 2.
You will also need to set up the CH7_OPTION and the MODE_CH. I used channel 5 (gear switch) for the CH7_OPTION and channel 6 (flaps) for the MODE_CH.
Your Flight Modes should be from top to bottom: Auto, Auto, Learning, Learning, Manual, Manual.
How have you connected your R/C receiver to the Pixhawk? Are you using a PPM encoder and a parallel output R/C receiver?
Remember, you need to Arm the Pixhawk with the push button before it will provide any PWM output on its servo output channels.
The above should help you get going with a PPM encoder and parallel output receiver. Your transmitter must support at least 6 channels.
TCIII ArduRover2 Developer