There are two methods of importing stencils into OmniGraffle: A manual method and an automatic method. You use the manual method if the stencils you are importing are not found on Graffletopia. Such stencils may be company-internal stencils, or even commercial ones.
This method works with any stencils, not only the ones found at Graffletopia, but for the sake of explanation we'll use Graffletopia as an example.
Lets say you need to annotate your diagram, but adding a lot of lines and text will clutter up the diagram. A common way to annotate a diagram is to use numbers or letters surrounded by a circle. In the accompanying text you can then explain in length about each annotated point.
You start by creating small circles with numbers inside. You find this extremely tiresome after a while, and ask yourself if there is a ready-made stencil available.
You go to the Graffletopia home page and enter numbers...