You need to have a long panel attached to one side of the robot. It needs to be one or two LEGO units above the balanced position of the elevator. Then your robot needs to pass slowly next to the elevator and push it with the panel until the elevator reaches its balanced position. The panel is used because it won't let the elevator get past the balanced position. The panel needs to be attached horizontally.
This mission is difficult because you can very easily lose the balance point and the elevator will fall.