You can only use maildir if you have dovecot or courier installed. This setting has never to be changed on a running server, it is only set once after you installed the server. Never change it afterwards or you will loose the access to all emails stored before on the server!
ANOTHER NOTE: After enabling MAILDIR, the ISPCONFIG interface no longer has the option to set MAIL maximum size for the users in their settings. I can only set WEB space allocations.
Thats fine and the intended behaviour as the mail quota of a user is always identically to the website quota, so the mailquota field is disabled.