This feature was originally meant to allow site visitors to add Calendar Manager events to their own personal Google Calendars with a single click. Due to changes that Google has made to their Calendar product this feature isn't working correctly anymore, so we had to deactivate it. Let's take a look at what happened, and how users can work around it.
What had been happening was that if a site user, let's call her "Alice," clicked "Add to Google Calendar," then Alice's Google Calendar popped up and the Calendar Manager event was duplicated on her own personal Google Calendar. So far so good: that's how it's supposed to work for everybody. The recent changes that Google made affected what happened to the next user who clicked that link.
Let's say that Bob comes along after Alice and clicks the same "Add to Google Calendar" link. Poor Bob doesn't get to add the Calendar Manager event to his Google Calendar. Instead, because Alice has already clicked that link, Google (for some reason) considers Alice's copy of the event to be the source. Bob is therefore directed to Alice's Google Calendar to get an iCal feed link from Alice's Google Calendar account. Since Bob doesn't have permission to access events on Alice's calendar, Bob encounters an error and is unable to add the event to his own Google Calendar. Bob ends up sad and frustrated.
Finally, users go to their own Google Calendar, click the down arrow next to "Other Calendars" on the left to expand the menu there, and select "Add by URL."