The Courier part is optional, you don't have to use it if you're happy with a POP3/IMAP daemon that uses mbox. If you want to have Maildir, then install Courier.
So you can
a) install Courier as per the instructions in the howto,
b) stay with your current setup and try to fix the POP3 problem. To do so, please have a look at /etc/inetd.conf and search for appropriate entries for POP3 and IMAP. You can also post your /etc/inetd.conf here so that we can see where the problem might be.