If you want it on your Add A Site page, try putting it in tables.php (make sure you are editing the one for the right theme) right after:

<!-- Start Form Table -->

For my site I changed it to:

$form_table = "\r\n\t\t<!-- Start Form Table --> <p align=\"center\" class=\"regularText\"><B>All Submitted Links Are Subject To Approval</b></p><p align=\"center\" class=\"regularText\">Please provide a Reciprocal Link by adding the following to your site and indicating the Reciprocal Link URL below.<br />
&lt;a href=\"http://www.YOURSITE.com\"&gt;YOUR SITE NAME&lt;/a&gt;</p>";

Just replace http://www.YOURSITE.com and YOUR SITE NAME.

If anybody knows of any reason not to put it there, please say so.

HTH,
John