I think you're missing a parenthesis after the 0 near the end.
mysql_query("insert into forumtopic (topic,text,cat,lastpost,originator,posts) values('".$topic."','".$text."','".$_POST['cat']."','".$lastpost."','".$seusername."','0')") or die(mysql_error());
Also the 0 shouldn't need quotes unless it's stored as VARCHAR or TEXT