There was an omission in the RadControls for ASP.NET AJAX Q2 2010 SP1 VSExtensions we solved shortly after that.
To fix this error I would recommend you to use the latest version of the Telerik WebUI VSExtensions
Which version of the Visual Studio IDE are you using?
If VS2010, you could use the steps below:
1) Close all Visual Studio instances
2) Use Windows Explorer to check the following two folders for Telerik subfolders and delete them:
3) Visit the Visual Studio Online Gallery and download the Telerik WebUI VSExtensions
to a folder of your choice
4) Start a Visual Studio 2010 Command Prompt As Administrator and navigate to the folder you downloaded the VSIX file to
5) Run the following command:
vsixinstaller /a [TelerikWebUIVSIX_FileName.vsix]
6) Repeat steps 3) -> 5) to get any Telerik VSExtensions you need added to your IDE.
If, on the other hand, you use Visual Studio 2008, you would need to visit your Telerik Account and download the RadControls for ASP.NET AJAX Q2 2010 SP2 installer (or, better yet, use the latest one, Q2 2011).
the Telerik team