Ajax Control Toolkit

Hi,

I have downloadloaded Ajax control Tool kit version 3.5 on my website - by creating a tab..

Now I dragged the calendar extender on my page, I received an error;

Reference required to assmebly 'System.Web.Extensions, Version=3.5.0.0, Culture-neutral,
PublicKeyToken=31bf3856ad364e35' containing the base class 'System.Web.UI.ExtenderControl'. Add one to your project.

Then this line is highligted;

<%@ Page Language="VB" MasterPageFile="~/site.master" AutoEventWireup="false" CodeFile="LoggedChangeReq.aspx.vb" Inherits="Mainpages_LoggedChangeReq" title="Updated Change Control Form" %

Please help - thanks