Gcal Sidebar pulls a Google calendar feed and displays it in the sidebar of your wordpress blog. It supports both widget and shortcode modes.
[gcal-sidebar feed_id="FEED_ID" ] is the bare minimum needed to display a calendar. Additionally, refer to the Description for a full list of shortcode options
Simply put the ID of the calendar, GCal Sidebar will generate the URL itself. You can get the ID by going to the settings of a calendar, then looking in the "Calendar Address" section on the far right.
This is somewhat harder. Again, go to the settings page of the calendar. Look at the bottom in the "Private Address" section. Click on iCal, and copy the text between 'private-' and '/basic.ics'. That is your private key. Paste that into the public / private config box to allow access to private calendars.
Get the Calendar ID, then go to the page and scroll down to the "Custom Fields" Section. Create a new field named gcal_sidebar_feed_id and put the calendar ID in as the value.