Telerik RadControls for ASP.NET are added automatically to the Visual
Studio .NET 2008 Toolbox during installation when using .NET
3.5. However if you are using
the ZIP distribution or EXE installation (when using .NET 3.0 or .NET
2.0 with Visual Studio 2008), you can do this manually. Have you tried the steps described in the documentation?
Adding RadControls to the Visual Studio 2008 Toolbox