The Power Plant is down and it needs to be fixed, this is achieved by opening the green carbon dioxide pump and adjusting the pipes to get the flow working properly. Use pipe numbers 6, 8, 1, 3 and 11.
When you have done that climb up the left ladder and use the pump controls to move the coin around until it rests on the circle and you can take it. Climb down the ladder and go to the red building by climbing up the other ladder. Insert the coin into the machine and press the four buttons so you automatically take a deflated balloon. Then climb up on the roof and use the balloon on the valve to fill it with helium.