When I converting my project from Visual Studio 2008 to Visual Studio 2010 I was getting this error in my webpage:
Unable to create type ‘telerik:RadPanelBar’. Could not load file or assembly ‘System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35’ or one of its dependencies. The system cannot find the file specified.
After searching and reading quite a fair bit on the net, I thought it might be that I have a version of the dll that is not compatibile with NET4.0 so I visited telerik.com accessed my account, downloaded the latest “RadControls for ASP.NET AJAX”, installed it and copied the generated Telerik.Web.UI.dll from the folder bin35 into my project bin and rebuilt. This made the error go! Please, keep in mind that even though I converted to VS 2010, I was still running on NET3.5, if you are running on version NET4.0 make sure to copy the dll from bin40 if available.