Can't get ModalPopupExtender to work

I am new to ASP.NET, and I am trying to use the Ajax Control Toolkit for the first time. I am trying to get a modal popup to work, but it will not show up. Here is the ASP code:

<%@ Page Title="Page1" Language="vb" MasterPageFile="~/Site.Master" AutoEventWireup="false"
    CodeBehind="Default.aspx.vb" Inherits="Project._Default" %><%@ Register TagPrefix="asp" Namespace="AjaxControlToolkit" Assembly="AjaxControlToolkit"%><asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent"><style type="text/css">
        #divError {
            height: 25px;
        }</style></asp:Content><asp:Content ID="MenuContent" runat="server" ContentPlaceHolderID="MenuContent"></asp:Content><asp:Content ID="YearContent" runat="server" ContentPlaceHolderID="YearContent"><br /><div id="divError" runat ="server" style="vertical-align: middle;"><table style="vertical-align: middle"><tr><td style="height: 5px">&nbsp;&nbsp;</td></tr><tr><td>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<asp:Label ID="lblError" runat="server" Font-Bold="True" ForeColor="#CC3300"></asp:Label></td></tr><tr><td style="height: 5px">&nbsp;&nbsp;</td></tr></table></div><asp:SqlDataSource ID="YearDS" runat="server"
        ConnectionString="<%$ ConnectionStrings:ConnStr %>"
        SelectCommand="GetData" SelectCommandType="StoredProcedure"></asp:SqlDataSource><asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"></asp:ToolkitScriptManager><asp:Button ID="btnMPE" runat="server" Text="Button" /><asp:Panel ID="PanelFY" runat="server" CssClass="popup" align="center" style="display:none"><iframe style=" width: 350px; height: 300px;" id="irm1" src="~/FiscalYear.aspx" runat="server"></iframe><br/><asp:Button ID="btnFYClose" runat="server" Text="Close" /></asp:Panel><asp:ModalPopupExtender ID="FYModalPopupExtender" runat="server"
        CancelControlID="btbFYClose" DropShadow="True" Enabled="True" PopupControlID="PanelFY"
       TargetControlID="btnMPE" BackgroundCssClass="background"></asp:ModalPopupExtender></asp:Content>

Here is the css:

.background
{
    background-color: Black;
    filter: alpha(opacity=90);
    opacity: 0.8;
}

.popup
{
    background-color: #FFFFFF;
    border-width: 3px;
    border-style: solid;
    border-color: black;
    padding-top: 10px;
    padding-left: 10px;
    width: 400px;
    height: 350px;
}

Thank you.