I believe the mailbox_command is what you use when you have procmail deliver mail to the appropriate folder within your Maildir. The procmail portion is setup in a .procmailrc (IIRC) in each users homedir. If you don't have this file, or have not setup procmail at all yet, then either read up on it quickly and institute it accordingly, OR comment out the line for mailbox_command and restart postfix. From there, your IMAP server, courier, should be setup to deliver the mail into your Maildir.
Check your /etc/courier-imap/imapd file to ensure your have:
Again, restart courier-imap afterwards.