As I see in the screenshot, your project's target Silverlight version is Silverlight 5. Please, make sure that the currently loaded distribution version in the Toolbox is the same as the version of your project references. Both the references and the Toolbox should use RadControls for Silverlight 5 distribution. As I already mentioned, Silverlight 4 distributions are versioned *.1040 and Silverlight 5 distributions are versioned *.1050.
You can check the version of the RadControls in the toolbox from the Toolbox Configurator (started from the Telerik menu, then RadControls for Silverlight -> Launch Toolbox Configurator). The current version is displayed on the right side.
Please, let me know if fulfilling these conditions restores the controls in the Toolbox.
Besides, we have identified the WCF Ria Services as a possible cause for the issue. Do you them installed?
the Telerik team
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