try this,
<asp:scriptmanager id="Scriptmanager1" runat="server" xmlns:asp="#unknown">
<asp:dropdownlist id="ddl1" runat="server" xmlns:asp="#unknown">
<asp:listitem>me
<asp:listitem>others
<asp:listitem>select
<asp:requiredfieldvalidator id="rfv1" runat="server" controltovalidate="ddl1" initialvalue="select" xmlns:asp="#unknown">
ErrorMessage="Please Give the Required Field">
<cc1:validatorcalloutextender id="v1" runat="server" targetcontrolid="rfv1" xmlns:cc1="#unknown">
CC1 is the extension that the AjaxLibrary uses