you have not added the "script manager". The script manager is mandatory to use any of the ajax controls. The AjaxContolToolKit library may not has registered with the page. Ajax Tool kIt may has not installed in your machine.
follow these steps--
--> Install Ajax Tool Kit
--> Reset your Visual Studio Tool Box
--> Add new tab in your tool box and name it as Ajax Extensions
--> Right click on your new tab - Click Choose Items
--> Browse your downloaded "AjaxControlToolKit.dll"
--> verify the selected controls
--> Click Ok
Now your new tab in tool box will be filled with ajax controls
Now first drag and drop "ScriptManager" in to your page source.
check the below line has added in your page at the top:
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
From now you can add any ajax controls..
Hope this helps..