why Rad Controls Design is not visible at design time in my solution?

I installed Rad Controls. Using that controls, I designed my application pages. At design time, the design is not visible. It shows the following error:

RadTabStrip1Failed to create designer 'Telerik.Web.UI.RadTabStrip, Telerik.Web.UI, Version=2010.1.415.35, 开发者_Python百科Culture=neutral, PublicKeyToken=121fae78165ba3d4'

How can I resolve this error?

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U can take a look to these links:

http://www.telerik.com/community/forums/aspnet-ajax/splitter/failed-to-create-designer.aspx

http://www.sitefinity.com/devnet/forums/sitefinity-3-x/developing-with-sitefinity/asp-net-ajax-radcontrols-and-sitefinity.aspx

http://blogs.telerik.com/atanaskorchev/posts/09-03-06/Meet_Telerik_Web_Design_dll.aspx

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Refer to the following KB regarding this issue:

Design-time Troubleshooting