Not bad.
I thought it was a bit slow, though. Watching that little guy slowly move around made me want a "just get it over with" button to go to the next thing.
About the first movement ActionScript: You have a "jumping" variable that sets whether the character is jumping or not. So "if (jumping == true) { vel_y -= 2;" means "If we're jumping, decrease y velocity (make us fall faster". And "if (Key.isDown(Key.SPACE) && !jumping)" means "If space is pressed AND we're NOT jumping". See, the "&&" means "and" and the "!" means "not". Simple, right?