You can simply connect third-party calendars by adding them in Apple’s default Calendar.app. Just open the default calendar, go the the menu and select “Add Account…” and select your account. After adding your calendar account there, you can close Apple’s calendar app and don’t have to use it anymore if you don’t want, your account is automatically registered with Meeter now and you will see your upcoming events in the menu bar!

For more information on how to do this, see Apple’s official support page