Modal Popup Help

This is first time i am using modal popup.

Steps i followed..

• I first get the ajaxtoolkit.dll and place it in the bin folder
• Added reference of ajaxcontroltoolkit to my project then added

  <form id="form1" runat="server">
  <ajax:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" ScriptMode="Release"
  AsyncPostBackTimeout="300"/>
  <asp:contentplaceholder id="cp_content" runat="server"></asp:contentplaceholder>
  </form>

to My master Page.(i am using Umbraco)

         Then i registered ajax in my ascx file, and kept the modal popup in the control

<ajax:ModalPopupExtender ID="mpeShowRecords1" runat="server" BehaviorID="mpeShowRecords1"
TargetControlID="lnktest" PopupControlID="pnlGridview1" BackgroundCssClass="modalBackground">
</ajax:ModalPopupExtender>

<asp:Panel ID="pnlGridview1" runat="server" CssClass="ModalWindow" Style="display: none;
overflow-y: scroll;" Height="540px" Width="900px">
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Literal ID="litIntroText" runat="server" Text="This is test for modal popup." />

<asp:LinkButton ID="lnkDeleteButton" CssClass="floatRight" runat="server" Text="Back" />
</ContentTemplate>
</asp:UpdatePanel>
<asp:LinkButton ID="btnDeleteOwnUserCancel" runat="server" CssClass="closeWindowIcon"
Text="Close This" />
</asp:Panel>

<asp:LinkButton ID="lnktest" runat="server" Text="Test" />

I am getting error like

Uncaught TypeError: Cannot read property '_behaviors' of undefined

Could some one please help me in this.. i spent days trying to figure this out.

Thanks

Dheeraj Palagiri.