You have three problems:
* How do you create a new object?
* How do you draw the objects?
* How do you move them?
I’ll start now and say that my Cocoa experience predates Leopard, is based on desktop Mac programming, and it’s probably getting mixed in with old-school Mac programming, but the general outline should be sound.
Somewhere in your model class should be an NSSet of objects that you can interact with. When you create an object, you should generate X and Y coordinates for it that place it off-screen. You should also have rules for how it moves – the range of possible delta-X and delta-Y values per tick.
Then you can use an NSTimer to trigger each tick. When you get the timer event, you go through your NSSet of objects, update the position of each one of them. Check for a collision with the user’s finger. If you don’t have one, trigger the screen NSView to redraw. If you do have one, do whatever is appropriate.
Then in your custom NSView subclass for that screen, write a method that goes through that magic NSSet and redraws each of them. There’s a method that you override, that only gets called when the superview determines its subviews should be redrawn.
Most of this info should transfer to the iPhone, from what I understand. It should at least give you a starting point.
Good luck.