RadTreeView is indeed located in the Telerik.Windows.Controls.Navigation assembly which should be installed on you computer. By default the WPF suite is installed in "C:\Program Files\Telerik\RadControls for WPF Q2 2008" and all assemblies are located in the Binaries sub-folder. If you have used the default settings of our installer you should be able to find the Navigation assembly in "C:\Program Files\Telerik\RadControls for WPF Q2 2008\Binaries".
I guess that in your second question you are referring to the application that shows the various examples of our controls and I can say that, for now, the source code of this application is not publicly available but we could possibly make the code available in the future.
the Telerik team
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