have you register the ajaxdll in your page ..
if no do it first
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<asp:TextBox ID="txtDOB" runat="server" class="datefd" />
<asp:ImageButton ID="imgcalnd" runat="server" Height="16" Width="16" ImageUrl="~/images/Calendar.png" />
<cc1:CalendarExtender ID="calcontl" runat="server" TargetControlID="txtDOB" PopupButtonID="imgcalnd"
Format="dd/MM/yyyy" />