You have to change these lines of code inside the text:
$output .= "Available from <a href='http://www.yoursite.com/' target='_top'>Your Link Output Keywords</a><p>";
<LINK rel="stylesheet" href="http://yoursite.com//store/skin1/skin1.css">
<LINK rel="stylesheet" href="http://yoursite.com//store/skin1/tabs.css">
<center>
<h1 style="background-image:url(http://www.yoursite.com/store/skin1/images/dialog_bg_n.gif)">Link to <?php echo $product; ?></h1>
</center>
Basically anywhere where it says "yoursite", needs to be modified
You can see working version of this mod on our site (right above the save 15%):
http://www.mailboxixchange.com/store/product.php?productid=18674&cat=0&page=1