Click on the 3 lines next to the search space and click on Calendars

Select the calendar you want to be added to and click add

If the calendar you want to be added to is not on this list, click on search for calendars then type the name of the calendar on the search box provided. Select the calendar you want to be added to once you see it and click on add.  

The calendars you are subscribed to will appear on the column on the right