This article will explain how to sync your 123-mail account with iCal on Mac OS X Lion and Calendar on OS X Mountain Lion.
Open iCal/Calender and select Preferences from the menu bar.
From the Preferences window, click on the Accounts tab.
Create a new Account by clicking on the Plus symbol in the bottom left of the accounts panel.
For the account type, select CalDAV then add your email address as the username. The password will be the same as your webmail password. Finally add the Server Address dav.123-reg.co.uk/
Please note: When adding the server address, it is important to add the forward slash at the end.
Your iCal/Calendar will now sync with your 123-mail account.