Are you trying to run the demos from the
Telerik\RadControls for ASP.NET AJAX Q1 2012\Live Demos\RadControlsExamples.sln?
Or maybe you just copy / paste the code from the online demos?
If so, note that they include a reference to the QuickStart namespace. Its definition / files can be found in the
\Telerik\RadControls for ASP.NET AJAX Q1 2012\Live Demos\App_Code\QuickStart
folder of your local installation.
Let me know if this helps.
the Telerik team
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