check below
<td>
<asp:TextBox ID="txtDOB" runat="server">
<atk:CalendarExtender ID="CalendarExtender1"
runat="server" PopupButtonID="imgCalendar" TargetControlID="txtDOB">
</td>
<td><asp:ImageButton ID="imgCalendar" runat="server" ImageUrl="~/Images/agt_action_success.png" /> </td>
i noticed that u have given wrong popupbuttonid property
PopupButtonID="imgCalendat" insted of PopupButtonID="imgCalendar"