If User Is Logged In, Else

<?php if ( $user_ID ) { ?>
<!-- enter info that logged in users will see -->
<!-- in this case im running a bit of php to a plugin -->

<?php mfp_display(); ?>

<?php } else {   ?>
<!-- here is a paragraph that is shown to anyone not logged in -->

<p>By <a href="<?php bloginfo('url'); ?>/wp-register.php">registering</a>, you can save your favorite posts for future reference.</p>

<?php } ?>