what do you mean?
the custom field has the name/url of the swf you want to load?
just in case, here’s a sample script
<head>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/swfobject/2.2/swfobject.js"></script>
</head>
<body>
<!--SOMEWHERE IN THE BODY-->
<div id="flashcontent"></div>
<!--AFTER THE LOOP-->
<?php if (get_post_meta(get_the_ID(), 'which_swf', true)) :
// the custom field name is 'which_swf'
// the value of this custom field is like "your-file.swf"
// the swf files are stored inside the 'custom' directory
$the_swf=site_url().'/wp-content/custom/'.get_post_meta($post->ID, 'which_swf', true);
?>
<script type="text/javascript">
var flashvars = {};
var params = {id: "flashcontent", wmode: "opaque", menu: "false", quality: "high", bgcolor: "#000000", allowFullScreen: "true"};
var attributes = {id: "flashcontent"};
swfobject.embedSWF("<?php echo $the_swf ?>", "flashcontent", "400", "200", "10", null, flashvars, params, attributes);
</script>
<?php endif; ?>
</body>
