The documentation uses a Crius V2 as example flight controller, but the Pin number is the same for all flight controllers.
By default MegaPirateNG uses PPM RC Input.
Connect your PPM RC Receiver to
Pin A8 on your flight controller.
If you want to use a non PPM RC Receiver you need to change your RC Input Type Configuration and connect the RC Receiver like this.
|A8||Yaw / Rudder|
|A9||Pitch / Elevator|
Connection for a 4 channels plane setup.