Someone shared a Google Calendar with me recently, and it showed up at calendar.google.com just fine. However, I could not see the shared events on my Mac (using Calendar) or on my iPhone or iPad (using the Calendar app).
To fix this, I had to visit https://www.google.com/calendar/syncselect directly, and check the shared calendar, then save the settings.
I'm posting this here because after spending an hour or so digging around all the Google Calendar settings online and settings on my Mac and iPhone, I finally found that link in some random forum post (nowhere to be found in Google's help docs!), and I don't want to forget how to manage shared calendar sync! Even using the iCal feed for the shared calendar didn't work.