Add onlinelist.php to your /my-plugins/ directory.
Add <?php show_online_users(); ?> wherever you want in your template. I suggest adding it to my-templates/front-page.php to the hottags like this:
<div id="hottags">
<h2><?php _e('Hot Tags'); ?></h2>
<p class="frontpageheatmap"><?php tag_heat_map(); ?></p>
<h2><?php _e('Online'); ?></h2>
<p><?php show_online_users(); ?></p>
</div>
Add <?php profile_last_online(); ?> to my-templates/profile.php. I suggest adding it under line 14 like this:
.