show the ModalPopupExtender depending on code behind

Hi
i would like to know how i can show/hide this ModalPopupExtender depending on session, NOT if the user just click on Sndmailtoadder button, please not that since add the panel or the popup ID's in the code behind its show me a redline under the ID name and with error message: The name '  ' does not exist in the current context 

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server"> <asp:UpdatePanel ID="adsdetailsupdat" runat="server"> <ContentTemplate> <br /> <asp:ListView ID="adsdetaillstview" runat="server" OnItemCommand="adsdetaillstview_ItemCommand"> <ItemTemplate> <asp:Panel ID="adsdetmsgreqpanel" runat="server" BackColor="White" Width="300" Height="90" CssClass=&quo ...