Demo

Look at Jaa's b2 blog.

Direction

1. Open b2comments.php or b2commentspopup.php template.

2. Include this code where you want the IP address to show up (remove ' * ').
IP logged: <*?php comment_author_IP() ?>

3. If somehow you want it invisible, you may have to use this code instead (remove ' * ').
You IP is logged. <div style="visibility: hidden"><*?php comment_author_IP() ?></div>

4. In both cases, the commenter's IP will appear in your admin panel as well.