If you have followed the default howto for all other tools, those will assume Maildir format. Which means you shouldn't have skipped these steps. Please execute those commands to complete your installation.
After restarting Postfix, I'm not sure if you have to re-create the mail account. That may be the case. Also sent an initial 'welcome' email to this account, or use the 'maildircreate' command to create the Maildir structure for this user. Postfix will automagically created the required directories, but the IMAP/POP servers will fail if the directory structure is not there.