AjaxControlToolKit Error: Element ToolkitScriptManager is not a known element

Hi,

I have added ajaxtoolkit along with all controls. Toolkit script manager alone showing not a kown element.

Please help me...Thanks in advance.

<asp:ToolkitScriptManager  runat="server"></asp:ToolkitScriptManager>
<asp:TextBox ID="txtEditor" runat="server" Width="300" Height="200" />
<asp:HtmlEditorExtender ID="HtmlEditorExtender1" runat="server" EnableSanitization="false"></asp:HtmlEditorExtender>
<asp:Button ID="Button1" Text="Submit" runat="server" OnClick="Submit" />
<br />
<asp:Label ID="lblContents" runat="server" />

 web.config as follows:

<configuration>
  <system.web>
    <compilation debug="true" targetFramework="4.5"/>
    <httpRuntime targetFramework="4.5"/>
    <pages>
      <controls>
        <add tagPrefix="asp" assembly="AjaxControlToolkit" namespace="AjaxControlToolkit"/>
      </controls>
    </pages>
  </system.web>
</configuration>