1. Click the Calendar tab along the left.
2. You should see a link within the left pane that says Open a Shared Calendar. Clicking that will open a new window entitled Open a Shared Calendar. If you know the name, you can type it in. Or you can click the Name button, which will open a list of the known names. Pick the one you need and click OK.
3. Click OK on the Open a Shared Calendar window.

You should now see the calendar in the right pane and the checked listing in the left pane.  If the calendar has not been shared, you will receive an error message and will need to contact the calendar owner to have them share the calendar with you.