'Maildir' is not an application which you need to install. Its format how mails are stored into your server.
As the guide itself says, if you have installed ISPConfig, just enable 'Maildir' as guided, ISPConfig would take care of next steps.
You can enable it into your /etc/postfix/main.cf with following code, just write following into your main.cf:
home_mailbox = Maildir/
then restart your postfix by
Hope this helps.