Please, make sure that you have added all required references as listed here. Note that there are different versions of RadControls for Silverlight - built for Silverlight 4 or Silverlight 5 respectively. The version of controls you are using must match the Target Framework of your project.
If you are not able to resolve the problem, we would appreciate it if you could open a support ticket and attach a sample project as a zip file there.
the Telerik team
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