You can check our OrgChart demo and its source code (it's almost the same for WPF (only style file differences). I also assume you have read our help articles: Diagram Layout Diagram Connection Routing
I also attached a sample project demonstrating a Tree Layout in a data-bound RadDiagram.
However, if none of these helps, could you please elaborate more on your scenario and on the issues you have in implementing TreeLayout ? Thank you in advance.