Choose which calendar you would like to add in the list above.
Login to your calendar.google.com account.
On the left above 'My calendars' you will see a plus sign to add other calendars, click that.
Select the 'From URL' option.
Copy the URL for your desired calendar as shown above, paste it into the space provided and click 'Add Calendar'.
And thats it, you should now see that calendar on the left under 'Other calendars'.
Turn on/off the different calendars using the checkboxes below