We are not sure what exactly the issue is when accessing the RadCoverFlow control in the Visual Studios designer. We noticed that you have added Width="600" Height="450" to the RadCoverFlowItem, make sure that the d:DesignHeight
of the MainPage are set correctly in order to see that content.
I recorded a short video for you showing how the scenarios runs on our side and that the contents of the RadCoverFlowItem are accessible via the designer.
Check out the new Telerik Platform - the only modular platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native apps. Register for the free online keynote and webinar to learn more about the Platform on Wednesday, February 12, 2014 at 11:00 a.m. ET (8:00 a.m. PT).