Hmm i can't test this code but u can try
in functions.php
//
// Update posts, topics, last_post, last_post_id and last_poster for a forum (redirect topics are not included)
// If $transaction == PUN_TRANS_END, this function will end the current transaction
//
function update_forum($forum_id, $transaction = 0)
{
global $db;
$result = $db->query('SELECT COUNT(id), SUM(num_replies) FROM '.$db->prefix.'topics WHERE moved_to IS NULL AND forum_id='.$forum_id) or error('Unable to fetch forum topic count', __FILE__, __LINE__, $db->error());
list($num_topics, $num_posts) = $db->fetch_row($result);
$num_posts = $num_posts + $num_topics; // $num_posts is only the sum of all replies (we have to add the topic posts)
$result = $db->query('SELECT last_post, last_post_id, last_poster, subject FROM '.$db->prefix.'topics WHERE forum_id='.$forum_id.' AND moved_to IS NULL ORDER BY last_post DESC LIMIT 1') or error('Unable to fetch last_post/last_post_id/last_poster', __FILE__, __LINE__, $db->error());
if ($db->num_rows($result)) // There are topics in the forum
{
list($last_post, $last_post_id, $last_poster, $subject) = $db->fetch_row($result);
$db->query('UPDATE '.$db->prefix.'forums SET num_topics='.$num_topics.', num_posts='.$num_posts.', last_post='.$last_post.', last_post_id='.$last_post_id.', last_poster=\''.addslashes($last_poster).'\', subject=\''.addslashes($subject).'\' WHERE id='.$forum_id, $transaction) or error('Unable to update last_post/last_post_id/last_poster', __FILE__, __LINE__, $db->error());
}
else // There are no topics
$db->query('UPDATE '.$db->prefix.'forums SET num_topics=0, num_posts=0, last_post=NULL, last_post_id=NULL, last_poster=NULL, subject=NULL WHERE id='.$forum_id, $transaction) or error('Unable to update last_post/last_post_id/last_poster', __FILE__, __LINE__, $db->error());
}
in index.php
// Print the categories and forums
$extra_sql = ($cur_user['status'] < PUN_MOD) ? ' WHERE f.admmod_only=\'0\'' : '';
$result = $db->query('SELECT c.id AS cid, c.cat_name, f.id AS fid, f.forum_name, f.forum_desc, f.moderators, f.num_topics, f.num_posts, f.last_post, f.last_post_id, f.last_poster, f.closed, f.subject FROM '.$db->prefix.'categories AS c INNER JOIN '.$db->prefix.'forums AS f ON c.id=f.cat_id'.$extra_sql.' ORDER BY c.disp_position, c.id, f.disp_position') or error('Unable to fetch category/forum list', __FILE__, __LINE__, $db->error());
AND THIS
// If there is a last_post/last_poster.
if ($cur_forum['last_post'] != '')
$last_post = '<a href="viewtopic.php?pid='.$cur_forum['last_post_id'].'#'.$cur_forum['last_post_id'].'">'.format_time($cur_forum['last_post']).'</a><br>'.$lang_common['by'].' '.pun_htmlspecialchars($cur_forum['last_poster']).'<br>'.$lang_common['in'].' '.pun_htmlspecialchars($cur_forum['subject']);
else
$last_post = ' ';
Now u must use something like phpmyadmin and add one field [ subject VARCHAR 255 null ] in forums table . i'm not 100% sure about this but should work's.
beacouse
, this happens beacouse when i copy code into guide i use my test forum where i have more mod's and now your forum try find this database values 
,meyby there is some problem .