Problem with versions of RadControls
Sitefinity uses 2010.3.1109.40 version, but instead the system uses the latest version 2011.1.315.40.
If you try to upgrade the project to the latest version, Sitefinity stops working...
If you use the 2010.3.1109.40, Visual Studio tries to use the other and produces errors at design time.
How we can make it work?
A default install should have the Telerik.Web.UI referenced in the bin folder, not your install folder, and while it may seem tempting to run the upgrade wizard I don't think that's a supported scenario :)
Grab the install version 1109.40 form the _EmptyProject install folder, drop it into your bin. Make sure the project references that, and all other projects also reference that DLL. Don't do that Add Reference thing and find Telerik.Web.UI from the .NET tab...
Or you can get ballsy and use the latest Telerik.Web.UI at your own risk with Assembly binding.