First of all, if you want help fast and useful, you should explain more what you need, and never ask someone else to do it for you. The point of the "How do I" forum is to learn by asking others for tips, not for the complete job.
There is no example called "catapult", but there is a lot of physics examples that could help you too.
That said, let's just think about what is a catapult and how to do it.
There are 2 part : the catapult and the boulder. Catapult is a simple sprite, playing an animation when you click the "lanch" button, or whatever the event is in your case. The boulder is a sprite too, with physics. When the catapult lanches it, just apply a force on the boulder, depending on the settings you want for the catapult.
Here is the simpliest capx I can think of to demonstrate this.