I'm not sure if it's the newest version but I downloaded it from here: http://svn.vizziweb.com/traq/trunk/inc/ticket.class.php
and there's a bug that doesn't allow you to add a ticket, in Ticket::check it checks if the user is logged in this line "if(empty($data['user_name']) && !$user->loggedin)" but there's no "global $user;" so $user is unknown and thus the error.
Thanks for reporting this, I will fix this when I get the time. Been a little busy.