This article presumes you have already set up a Google account and calendar, and have successfully synced it to 17hats.
In OSX go to "System Preferences", and select "Internet Accounts".
Select Google from the list of Accounts.
You will be prompted to login to your Google Calendar, please use the same log-in as the Google Calendar you have connected to 17hats.
Once connected, check "Calendars".
The next time you open Calendars app, your Google Calendar that is synced to 17hats will appear.
KeywordSearch: OSX Calendars, Calendar App, Set up, Connecting OSX Calendar to 17hats, ICal, ICloud, ICalendar