This is a bug in our documentation. RadDiagramPrintPreview and RadDiagramPrintPerviewDialog are only available in WPF. In Silverlight we use a native Silverlight print dialog that could be seen in our Diagram, RichTextBox demos. We cannot implement the features in the RadDiagramPrintPreview for SL due to framework restrictions in Silverlight.
Please excuse us for the inconvenience caused.
the Telerik team