This requires VPOP3 v6 or later. The following instructions are for Mac OS X v10.7.5 but should work with similar versions.
We have not been able to create an account by going to System Preferences → Mail, Contacts and Calendars → Add Account. That option simply comes up with errors that you can't get past.
Unfortunately, although Apple often do a good job with usability, this is a case where it falls down if Mac OSX doesn't recognise the server in use, and things become a lot harder than if they had just asked you to enter the details manually in the first place.
So, to add a CalDAV account, open the iCal program, and go to iCal → Preferences → Accounts. Press the + button at the bottom left of that page.
In the Add an Account page that opens, choose CalDAV from the Account Type box. In the User Name and Password boxes enter the VPOP3 username and password, and in the Server Address box enter the address of the VPOP3 computer - eg 192.168.1.1
Press Create. You will probably get some errors, possibly about being unable to verify the identity of the server or not being able to detect settings. Press Continue as necessary.
This setting method will allow you to continue to the actual account configuration. The Account Settings page will probably be completed with the details you entered previously, but check them just in case.
Press the Create button. You may get another warning about Server with Secure Communication Unavailable. Press Continue again (to prevent this warning you can configure an SSL certificate in VPOP3, if you have VPOP3 Enterprise). If you get taken back to the Account Settings page, press Create again.
You should, hopefully, eventually end up at a page called Calendar Server Options.
On this page you can now enter the details which iCal could have asked for in the first place.
Press the Create button
Hopefully this should now succeed, and if you click on the Calendars button in iCal, you should see your available calendars listed there.
Changes with the authentication system in VPOP3 v6.3 may cause issues with initial authentication with iCal. This should be fixed in v6.4