Unrecognized tag prefix or device filter (Visual Studio 2010)

[KhD]

I am trying my hands on Visual Studio 2010 (Asp.Net 4.0) for the first time. I want to use Ajax controls, mainly ComboBox control.

I followed the step-by-step procedure mentioned in this link to download and install AjavControlToolkit.

I also followed this link for steps to add ComboBox to my webpage.

But no matter what I do, there is a green line under the control and I'm getting the error "Unrecognized tag prefix or device filter 'ajax'".

My web.config file looks like this:

 <configuration>
       <system.web>
    <pages>
        <controls>
            <add tagPrefix="ajax" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit"/>
            <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
            <add tagPrefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
        </controls>
    </pages>
    <compilation debug="true" targetFramework="4.0">
        <assemblies>
            <add assembly="System.Web.Extensions.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
            <add assembly="System.Design, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
            <add assembly="System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/></assemblies></compilation>
  </system.web>
    </configuration>

The aspx page looks like this:

<form id="form1" runat="server">
       <div>
        <asp:ScriptManager ID="sm" runat="server"></asp:ScriptManager>
        <ajax:ComboBox runat="server" AutoCompleteMode="SuggestAppend">
          <asp:ListItem>ddd</asp:ListItem>
          <asp:ListItem>fff</asp:ListItem>
          <asp:ListItem>gggg</asp:ListItem>
          <asp:ListItem>hhhhh</asp:ListItem>
        </ajax:ComboBox>    
       </div>
    </form>

I have even tried deleting the schema files from C:\Documents and Settings\xxx\Application Data\Microsoft\VisualStudio\10.0\ReflectedSchemas location.

Please help me in successfully implementing the Combobox control. Thanks!

Edited:

As a different approach, I created a new website and installed the latest version of AjaxControlToolkit using NuGet. I rebuilt the solution. I still could not see the Ajax controls in intellisense. Is there anything else that needs to be done to implement it? Please help!!