Try this:
Open:
newthread.php
find:
$db->query("INSERT INTO ".TABLE_PREFIX."threads (tid,fid,subject,icon,uid,username,dateline,lastpost,lastposter,views,replies,visible, prefix) VALUES (NULL,'$fid','$subject','$icon','$author','$username','$now','$now','$username','0','0','$visible', '$prefix')");
Change to:
$db->query("INSERT INTO ".TABLE_PREFIX."threads (tid,fid,subject,icon,uid,username,dateline,lastpost,lastposter,views,replies,visible, prefix) VALUES (NULL,'$fid','$subject','$icon','$author','$username','$now','$now','$username','1','0','$visible', '$prefix')");
i haven't tested it, but i think it should work