ASP.NET VS2008 Ajax Problem

<asp:ScriptManager ID="ScriptManager1" runat="server"/>

<ajaxToolkit:FilteredTextBoxExtender ID="Filter1" runat="server" ValidChars="AEIOU" TargetControlID="TextBox1"></ajaxToolkit:FilteredTextBoxExtender>

<pages>
  <controls>
    <add tagPrefix="ajaxToolkit" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit"/>
  </controls>
</pages>