You need to delete the files from the locations specified in the blog post (depending on whether you develop under .NET 2.x or .NET 3.x framework) rather than deleting cookies or browser cache.
Furthermore, verify that you have a valid reference to the proper version of the Telerik.Web.UI.dll/Telerik.Web.Design.dll based on the framework version you have (those from the /bin
folder of the installation are for .NET 2.x and those from the /bin35
folder are for .NET 3.5).
Finally, restart your Visual Studio to ensure that the latest updates for design-time references will be applied.
the Telerik team