Accordion extender issue

This is my first time using Accordion and AccordionExtender, it would be appreciated if you can shed me some light on this.

Thank you very much.

I know there have been lots of discussion on this but mine seems to be unique:

I am using VS2008, ajaxtoolkit is 3.5.50508

I use Accorion in a tab container, the accodion is inside an UPDATEPANEL (which seems to be issue to lots of discussion), I have no issue viewing the accordion in design view and I am also able to compile the code with no error.

When I run it, it throws error: Parser Error Message: Unknown server tag 'asp:AccordionExtender'.

Here is my code:

<cc1:TabPanel ID="TabPanel2" runat="server"><HeaderTemplate>BNS Logical Access Review</HeaderTemplate><ContentTemplate><asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate><input type="button" onclick="Expand();return false;" value="Show All"  /><input type="button" onclick="Collapse();return false;" value="Collapse All"  /><br /><cc1:Accordion ID="Accordion1" runat="server"
            SelectedIndex="0" FadeTransitions="true" FramesPerSecond="40"
            TransitionDuration="250" AutoSize="None" ContentCssClass="Content" HeaderCssClass="HEADER"
             Width="80%"  ><Panes><asp:AccordionExtender ID="Accordion1_AccordionExtender" runat="server"
                Enabled="True" TargetControlID="Accordion1"></asp:AccordionExtender><cc1:AccordionPane ID="AccordionPane1" runat="server">

I also tried put

<asp:AccordionExtender ID="Accordion1_AccordionExtender" runat="server" 
                Enabled="True" TargetControlID="Accordion1"></asp:AccordionExtender>

right after

<cc1:Accordion ID="Accordion1" runat="server" 
            SelectedIndex="0" FadeTransitions="true" FramesPerSecond="40" 
            TransitionDuration="250" AutoSize="None" ContentCssClass="Content" HeaderCssClass="HEADER"
             Width="80%"  >

and before

<Panes>

it produces a slightly different error:

Parser Error Message: Type 'AjaxControlToolkit.Accordion' does not have a public property named 'AccordionExtender'.