I am not sure why the Items collection is empty in your case. However, you can take advantage of the predefined shape geometries and apply them either in xaml or in code-behind. Then in order to change the appearance of the shapes you can apply an explicit style on the shapes, or define a RadDiagramShape style through the RadDiagram.ShapeStyle property or you can define an implicit style for all RadDiagramShapes.
Also, please note that the RadDiagram exposes a Shapes collection that contains all shapes defined in it, while the items collection contains the RadDiagramConnection elements as well.
I attached a sample solution, please have a look at it and let me know if it helps or if I'm missing something.