Add AjaxControlToolkit and AjaxExtensionsToolbox to the references,
Modify your web.config file
<controls>
<add tagPrefix=”ajaxToolkit” assembly=”AjaxControlToolkit” namespace=”AjaxControlToolkit”/>
<add tagPrefix=”asp” namespace=”System.Web.UI” assembly=”System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35″/>
<add tagPrefix=”asp” namespace=”System.Web.UI.WebControls” assembly=”System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35″/>
</controls>
Add
<asp:ScriptManager ID=”scriptManager” runat=”server” EnablePartialRendering=”true” EnablePageMethods=”true”>
</asp:ScriptManager>
under your FORM tag
