The problem with walkers in SimplePlanes, and any game with fixed controls, is how do you make them move in the right fashion without any code, and using parts that have limited function? Here, the hips are set to analog vertical and the knees are analog horizontal. As you rotate the stick, the leg contracts, knee contracts, leg extends, knee extends, causing the walking motion. I've done this in Trials, but here there is nothing to really hold it upright, like the balloons in Banjo: Nuts & Bolts.

At any rate, here is how to control the silly thing. You will need easy access to Roll, Yaw, Pitch and VTOL; I recommend dual-joysticks. Begin by holding the analogs in opposite directions, then simultaneously rotate them at a steady pace clockwise for forwards, and counter for reverse.

Throttle: Activates Thrusters holding it all down.
Activate #1 to enable very slow turning with both sticks.
Blue joints (Hips): Pitch
Red joints (Hips): VTOL
Green joints (Knees): Roll
Yellow joints (Knees): Yaw

BONUS:
There is a 1 in 4 chance something fun will happen:
> Most often works best going forwards
> Somewhat often best going backwards
> Periodically will dance a jig in place
> Occasionally the feet will explode in a blaze of confetti.