By default, we only add the .NET 3.5 versions of our controls in the Visual Studio 2008 toolbox. This means that if your project is set to use .NET 2.0, you will not see the RadControls in the toolbox. You will need to manually add the .NET 2.0 version of the controls in order to use them with VS 2008. Here is how to do that:
Open the toolbox, right click inside and select "Add tab". Name the new tab 'RadControls for ASP.NET AJAX .NET 2.0". Then right-click inside it and select "Choose Items...". When the popup window opens, make sure that the ".NET Framework Components" tab is selected. Then click the "Browse..." button. Navigate to "C:\Program Files\Telerik\RadControls for ASPNET AJAX Q1 2008\Bin" and select the Telerik.Web.UI.dll
file. Finally click OK. After these steps you should be able to see the RadControls for ASP.NET AJAX.
the Telerik team