There is on forum, try to search. I've it but now my connection is too slow. If you don't find it, contact me by pm and I send it to you.
However is easy. Use physic behavior for catapult, green arrow (i don't know real name in english) and the objects. Set a revolute joint to arrow for catapult and try to put your object on it! I use physic for move my objects, but, depends on your game, you can use drag&drop to move them.