Using Telerik.Web.Ui; not working

Question

Hi !! I am quite new to .NET and using Visual studio 2008. I have downloaded and installed Telerik web controls also known as Rad controls for ajax. As per the instruction in documentation I am including following library "using Telerik.Web.UI;" but it gives the following error.

The type or namespace name 'Telerik' could not be found (are you missing a using directive or an assembly reference?)  

I have reinstalled visual studio and RAD control twice but getting the same error.

Please anybody guide me where i did the mistake?
Thanks