Redireccionar a otra pagina si no encuentra Flash

Deberias usar el siguiente script:
<script>
<!--
//Cambia la variable inferior con tu URL
var sURL="http://direccion.url/pagina.htm"
var agt=navigator.userAgent.toLowerCase();
var ie = (agt.indexOf("msie") != -1);
var ns = (navigator.appName.indexOf("Netscape") != -1);
var win = ((agt.indexOf("win")!=-1) || (agt.indexOf("32bit")!=-1));
var mac = (agt.indexOf("mac")!=-1);
if (ie && win) { pluginlist = detectIE("ShockwaveFlash.ShockwaveFlash.1","Shockwave Flash"); }
if (ns || !win) {
nse = ""; for (var i=0;i<navigator.mimeTypes.length;i++) nse += navigator.mimeTypes.type.toLowerCase();
pluginlist = detectNS("application/x-shockwave-flash","Shockwave Flash");
}
function detectIE(ClassID,name) { result = false; document.write('<SCRIPT LANGUAGE=VBScript>\n on error resume next \n result = IsObject(CreateObject("' + ClassID + '"))</SCRIPT>\n'); if (result) return name+','; else return ''; }
function detectNS(ClassID,name) { n = ""; if (nse.indexOf(ClassID) != -1) if (navigator.mimeTypes[ClassID].enabledPlugin != null) n = name+","; return n; }
pluginlist += navigator.javaEnabled() ? "Java," : "";
if (pluginlist.length > 0) pluginlist = pluginlist.substring(0,pluginlist.length-1);
if (pluginlist.indexOf("Flash")!=-1)
window.location=sURL;
//-->
</script>