It depends a bit on what the broader context is, are you using MVVM, do you have geometries in some other assembly...?
The most basic example would be the following. You set somewhere in XAML or in code the DataContext of the shape (or of some object in the hierarchy of the shape, this could be the RadDiagram instance itself, or like below the embracing Window) and then databind to a property which returns some geometry.
then in XAML you can do something like
Note that if you want custom geometry you can use Blend or even a standard SVG editor to create some geometry in string format (stuff like "m99,138l135,-48l94,93l-96,114l-32,-106l-128,46l27,-99z") and then use the GeometryParser to return the Geometry;
If you want to use an enum type then you can databind as well but you'll need a value converter and plug it into the XAML syntax.
Hope this helps, Fr.