If this is an old contrib i think that inside the addon file you should remove these lines on top
<!doctype html public "-//W3C//DTD HTML 4.01 Transitional//EN">
<html <?php echo HTML_PARAMS; ?>>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=<?php echo CHARSET; ?>">
<title><?php echo TITLE; ?></title>
<link rel="stylesheet" type="text/css" href="includes/stylesheet.css">
<script language="javascript" src="includes/general.js"></script>
<script language="JavaScript" src="includes/javascript/spiffyCal/spiffyCal_v2_1.js"></script>
<link rel="stylesheet" type="text/css" href="includes/javascript/spiffyCal/spiffyCal_v2_1.css">
</head>
<body marginwidth="0" marginheight="0" topmargin="0" bottommargin="0" leftmargin="0" rightmargin="0" bgcolor="#FFFFFF">
<!-- header //-->
<?php require(DIR_WS_INCLUDES . 'header.php'); ?>
<!-- header_eof //-->
and remove these lines
<!-- left_navigation //-->
<?php require(DIR_WS_INCLUDES . 'column_left.php'); ?>
<!-- left_navigation_eof //-->
and remove also
<!-- footer //-->
<?php require(DIR_WS_INCLUDES . 'footer.php'); ?>
<!-- footer_eof //-->
then add after: require('includes/application_top.php');
require(DIR_WS_INCLUDES . 'template_top.php');
and add to the end before: require(DIR_WS_INCLUDES . 'application_bottom.php');
require(DIR_WS_INCLUDES . 'template_bottom.php');