Skeletron uses three cylinders, two to grip the rope and one main cylinder that's used to carry out the climb. The top and bottom grips work alternately. As the bottom grip locks, the top grip releases and the main cylinder then pushes the legs down, extending out to push the steel chassis upwards. This alternate motion of extending and retracting the legs is what gives Skeletron its climb.
Follow the instructions in the animation to make Skeletron climb. You will need to remember to lock the upper grip on the rope before releasing the lower one or Skeletron will crash to the ground. If you don't release at least one of the grips the robot won't move. You've got just enough CO2 to get to the top so use it sparingly.