Thanks for the provided information.
The issue that you experience is related to the fact the Visual Studio cannot resolve the Telerik.WinControls.UI.Design assembly. You should either add this assembly in your GAC, or add a reference to this assembly in your project. The first option is preferable, because if will affect positively all projects that use RadControls for WinForms.
I hope this helps.
the Telerik team
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