AJAX help needed: ModalPopupExtender

I've been through about 20 different AJAX ModalPopupExtender examples now, and I can't get a single one to work. I've registered my assembly reference:

Code:

---

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="act" %>

---

I've put everything inside a form and have the corresponding panel and button(s) setup:

Code:

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<form id="form1" runat="server">
<asp:scriptmanager id="ScriptManager1" runat="server">
</asp:scriptmanager>
   
<asp:Button runat="server" ID="btnEditCustomer" Text="Edit text" />
<asp:Panel runat="server" ID="pnlEditCustomer">test message</asp:Panel>

<act:ModalPopupExtender ID="ModalPopupExtender1" runat="server" 
TargetControlID="btnEditCustomer"
PopupControlID="pnlEditCustomer"
BackgroundCssClass="modalBackground"
OkControlID="editBox_OK"
OnOkScript="yes()" />

---

The button appears on my page, but nothing happens when I click on it. I'm using VS2008, outputting to Firefox 3.6.16 and have the latest AJAX Toolkit installed (VS2008 allows me to drag and drop the controls without any problems or compiler errors).

Hello,

It is most likely that there is some form of JavaScript error on the page.

Have you checked the JavaScript console to see what is going on?

Gary