The AR6400 is advertised as an aircraft micro receiver & motor controller. With a couple of exceptions, the reversing of aircraft engines (in flight) is considered a Bad Thing (tm).
If you want forward/reverse motor control, you need to get an ESC designed for ground vehicles, cars/tanks/etc., or possibly for boats, with reverse (not all of them have it). Or use a spare servo and a DPDT switch. Or build an electronic reversing switch. Or you could combine all the functionality by using a little MCU to read the throttle, pick mid-position as "0", and then drive the motor forward and reverse with an H bridge. -- For starters (way too big for anything besides proof of concept) look at an Arduino and a motor shield (eg. Adafruit's. Sparkfun might have a motor shield too. Dunno.).