You need a div called "flashcontent" for SWFObject to put the movie in.
Find this in your code:
<script type="text/javascript"> var so = new SWFObject("mainMenu.swf", "mymovieName", "100%", "100%", "6", "#336699"); so.write("flashcontent"); </script>
...and put this BEFORE it: <div id="flashcontent">This is replaced by the flash movie</div>