I've just done a test, and created a new user in ISPconfig.
When the new users logsin on Round Cube, he does have a Trash folder!
Did you use Hans his howto? (http://www.howtoforge.com/roundcube_webmail_ispconfig
If so did you change the needed settings in the "main.inc.php"
// display these folders separately in the mailbox list
$rcmail_config['default_imap_folders'] = array('INBOX', 'Drafts', 'Sent', 'Junk', 'Trash');
Here is some more info: http://www.howtoforge.com/forums/showthread.php?t=2081