To get started with Contacts and Calendars, please log in to Fastmail, the interface for webmail, calendars and contacts. This will set up your initial calendars and address books, which you can then rename, remove or add to, as it suits you.
Once your contacts and calendars are created, you can access them through webmail or any calendar/address book program that supports DAV.
Your username should be the entire email address you used to log into webmail. For example, user@pobox.com.
Your password should be your Pobox password, or, if you are using 2-factor authentication, an app-specific password.
For calendars, please enter the server caldav.fastmail.com/. Your program should automatically detect all the calendars set up on your account. Please make sure you are adding this as a new account, not a new calendar subscription.
For address books, please enter the server address carddav.fastmail.com/
Step-by-step instructions with screen shots for specific email clients can be found in the Fastmail help pages. Your Pobox address should be used anywhere it specifies your Fastmail username.