ajax control is not showin date when i click over textbox

hi

 below is my html code

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<asp:Content ID="mastreJob" runat="server" ContentPlaceHolderID="MainContent">
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
        <asp:TextBox ID="txtDate" runat="server"></asp:TextBox>

        <cc1:CalendarExtender ID="CalendarExtender1" TargetControlID="txtDate" runat="server">

        </cc1:CalendarExtender>
but calender control is not showing when i click over textbox for date selection

pls help