If you haven't already, you will need to first check out: How do I sync my tasks to my calendar?
1. Open up iCal on your computer. It should be located in your Applications folder.
2. Click 'Subscribe...' which is under 'Calendar' in the top menu:
3. In the following box, type in your unique 'Get It Done' calendar address. This link is located in your 'Calendar Sync' tab within your Get It Done settings. If you do not know where to find this, you should first read: How do I sync my tasks to my calendar?
And that's it! Your tasks should now show up in your Apple OS X iCal. Click 'Refresh' to manually update your calendar or 'Get Info' to change the Auto-refresh rate: