world pay is a remote checkout method so SSL is not really needed.
if you want to add it, you can either use an htacccess rule to redirect all traffic to the https address:
or you can add code to the pages created by the checkout wizard to redirect them to the https address:
<?php if((!(isset($_SERVER['HTTPS']))) || ($_SERVER['HTTPS'] == "") || ($_SERVER['HTTPS'] == "off"))header("Location: https://www.<your domain here>" . $_SERVER['PHP_SELF']); ?>