Ajax Calender Extender Doesn't Work

I know this is a silly question, bu I just can't get the ASP.Net Calender Extender work. I am looking at this problem for hours now.

I've downloaded the code from the Asp.net Ajax website and I've added the dll to my toolbox.

This is the code of my page

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:CalendarExtender TargetControlID="TextBox1" runat="server"></asp:CalendarExtender>

When I click the TextBox, it behaves like a normal TextBox. It doesn't show the calendar.

Is there something I forget, or is there something I am doing wrong?