Do the following :
1-Add the Ajax dll.
2-add this in your page :
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
3- Add the calender extender like this one :
<cc1:CalendarExtender ID="txt_NewsDate_CalendarExtender" runat="server" TargetControlID="txt_NewsDate">
</cc1:CalendarExtender>
4-Dont Forget to add this :
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
Good Luck.