The best way to do that is to use the body tag's onload handler to change the location of your menu page with JavaScript.
Many times, menus and pages on sites like yours may end up becoming disjoited when, say, the user clicks the back button, as the locations of the menu and main page will be in order in the browser's history. At least with an onload, the menu will always match the main page when it is displayed
If the name of your menu frame is "menu", then you can simply put this in your body tag:
<body onload="top.menu.location='menupage.html'">
