Sorry, it seems it's in index.php =o
Anyways, this is the one:
// Print the categories and forums
$result = $db->query('SELECT c.id AS cid, c.cat_name, f.id AS fid, f.forum_name, f.forum_desc, f.redirect_url, f.moderators, f.num_topics, f.num_posts, f.last_post, f.last_post_id, f.last_poster FROM '.$db->prefix.'categories AS c INNER JOIN '.$db->prefix.'forums AS f ON c.id=f.cat_id LEFT JOIN '.$db->prefix.'forum_perms AS fp ON (fp.forum_id=f.id AND fp.group_id='.$pun_user['g_id'].') WHERE fp.read_forum IS NULL OR fp.read_forum=1 ORDER BY c.disp_position, c.id, f.disp_position', true) or error('Unable to fetch category/forum list', __FILE__, __LINE__, $db->error());
That can be changed to:
// Print the categories and forums
$cid_sql = isset($_GET['cid']) ? "c.id=".intval($_GET['cid']) . " AND" : '';
$result = $db->query('SELECT c.id AS cid, c.cat_name, f.id AS fid, f.forum_name, f.forum_desc, f.redirect_url, f.moderators, f.num_topics, f.num_posts, f.last_post, f.last_post_id, f.last_poster FROM '.$db->prefix.'categories AS c INNER JOIN '.$db->prefix.'forums AS f ON c.id=f.cat_id LEFT JOIN '.$db->prefix.'forum_perms AS fp ON (fp.forum_id=f.id AND fp.group_id='.$pun_user['g_id'].') WHERE '.$cid_sql.' fp.read_forum IS NULL OR fp.read_forum=1 ORDER BY c.disp_position, c.id, f.disp_position', true) or error('Unable to fetch category/forum list', __FILE__, __LINE__, $db->error());
Then, for a certain category, use this link:
index.php?cid=1 (change 1 to category id)
*goes to add it in the wiki*