Add By URL
OK, click Add then select Add by URL
These are the three feeds I have for you... pick one and copy ALL of the tiny text:
WHS Calendar
http://www.google.com/calendar/feeds/2c054n5ehkqcqih12ijes69hl8@group.calendar.google.com/public/basic
WHS Sports Calendar
http://www.google.com/calendar/feeds/p4tppap404hpjd23hvd6m7i7i0@group.calendar.google.com/public/basic
Now paste the calendar feed into the URL box and click Add Calendar: