This line can be placed in the constructor of the MainWindow of your application. Or wherever the RadPanelBar control is used. For example if you have a Window with a PanelBar placed in its Xaml file you can set the ItemsSource property in the constructor of the window.
In addition I attached a sample project based on the How to Display Hierarchical Data help article. Please let me know if you need any further assistance.
Regards,
Martin
Telerik
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