How do I sync my contacts with MacOS?

Open “System Preferences” in Mac Contacts and click “Internet Accounts”

Click “+” in the bottom left hand corner to add a new account.

Choose “Add Other Account” from the end of the list and then scroll down and select "CardDAV Account"

Once you've selected CardDAV account, in the drop down box select "Manual". Then type your username, password and the server address of your Contactzilla address book (for “Server Address” enter the URL of the Contactzilla address book you wish to sync, without the ‘https://’ part (e.g. mac-contacts.contactzilla.com). 

Click “Sign In" and then ensure you have ticked the box "Enable this account".

The sync may take a few minutes. When the sync is complete, you will be able to see your Contactzilla address in Mac Contacts.