So... the shape selectors (I think you're referring to the finger guides (two sets of concentric circles on your canvas) are to move your shapes around when you drop them onto the canvas. So, if, let's say, you tap on a circle in the Shape menu. it will land on your screen along with the finger guides. You can use the guides to move the circle around and then once it's where you want it to be, you double tap inside the shape (the circle in this case) and it will "stamp" it onto the canvas.
If the shape is not the right size to encircle your image, you can enlarge it by placing two fingers on either side of the outline and pinching/spreading your fingers.
You can also tap on either one of the concentric circles to scroll through a circle, a square, a triangle, and straight edges. The x in the center hides the guides and whatever shapes haven't been stamped onto the canvas.
And just for fun... When you have a circle, a square or a rectangle on the canvas, you can grab one edge with a finger and drag to create an oval, a rectangle or a trapezoid (?).
Does that help?