At the beginning of the level, throw pesterballs at the squirtles to make them leave the river. Further down the river you'll find the squirtles sunning themselves at the base of a steep hill. At the top is a mankey. As you pass the squirtles, throw pesterballs at them, causing them to roll up the hill. If your timing is right, the mankey will fall down and land near the switch. As you approach the switch, throw another pesterball at the mankey, knocking him backwards onto the switch.