You are trying to modify the Items collection, while you should be using the GraphSource instead. You have to add items (nodes or links) in the graph source and not in the RadDiagram.Items. If you are trying to edit the diagram through its UI, then this is not supported with the beta release. However, for Q2 (the official release) we have prepared a bi-directional MVVM mechanism that allows this. If you modify the diagram through the UI, it will notify the GraphSource that a new item has to be created, added or removed from it. Stay tuned for this.
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>