Adding A External Calendar

I've added external calendars before.  Fairly simple, find the Add option when in Calendar App and select source.  I want to Import an Internet Calendar and when that option is clicked on a dialog box opens with a field to enter the URL of the Calendar.  I have a Google Calendar's URL, which I paste in the blank field but I don't see any new caledar added.  Thouoghts?  Suggestions?  Thank you.