Open your Google Calendar within a web browser.
Click on the Other Calendars dropdown menu link (to the right of Other Calendars)
Select "Add By URL" to get the following screen.
Insert http://c.caltrics.com/{youruniquecode} as your personalised address.
Optionally, to make this google calendar available to other platforms, like a BlackBerry 10, you should tick the box to say "Make publically accessible?" as in the following screen shot.
Then click Add calendar and you are complete.