Yep it's just after line 109 in wp-bulletin-board/php/wpbb-index.php e.g. just after line 109 but before 110/111 you could do this:
<p class="wpbb-centered">My content here </p>
Then to add a Login and Register link to those links add these two lines after line 115 if you made the above edit or 114 otherwise:
| <a href='<?php echo wp_login_url(); ?>'><?php _e('Login', 'wp-bb'); ?></a>
| <a href='<?php echo site_url('wp-login.php?action=register'); ?>'><?php _e('Register', 'wp-bb'); ?></a>
That should do it. Please let me know if you need any further help with this. Thanks!