That's interesting, basically, even if the current user is part of the local administrators group, if the User Account Control is turned on, the processes are run with limited privileges. That's the reason the ToolboxConfigurator (part of the VSExtensions) asks for administrative permissions. The problem you are experiencing is a strange one though, for the ToolboxConfigurator is operating in the local user registry, which the IDE has access to. On the other hand, a right-click on the Visual Studio icon and a click to the "Run As Administrator" option should not cause errors, it should launch the process... Can you check with the administrators in your organization for assistance with the machine configuration?
Anyways, the assembly, containing RadControls for ASP.NET AJAX is Telerik.Web.UI. You could browse it to manually populate the Toolbox. If you used the installer, you can find it under the RadControlsInstallDir (typically C:\Program Files (x86)\Telerik\RadControls for ASP.NET AJAX Q2 2011\BIN40). On the other hand, if you had used the VSExtensions to download the latest version, you could find the assembly under the %appdata%\Telerik\Updates folder.
Let me know if that helps.
the Telerik team