Hello Mattias ,
You can find the "Features" SL project in the "Features" folder located in the installation folder of RadControls. The path should be;
C:\Program Files (x86)\Telerik\RadControls for Silverlight Q1 2012\Demos\Examples\Features
You have to build this project and use it as assembly. Please let us know if you need further assistance.
the Telerik team
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