First, have you set the Calendar to be shared?
When in your Calendar (calendar.google.com), you need to go into the settings for that specific Calendar and click on "Share this Calendar".
This will give you the option to share it to SPS.org (which will require a teacher to be logged in to view it), or share it to the Public (no login required). You need to check the box next to "Share this calendar" before the others will become accessible.
Be sure to click "Save" in the top left corner before exiting the page.
After setting the Calendar to be accessible via sharing, you will need the address directly to the Calendar.
Click on the "Calendar Details" tab in the top left corner. (You can also click on "Calendar Settings" in the first menu if you exited out of the Calendar setup pages.)
On this page, there is a section for "Calendar Address." You will want to click on the blue "HTML" button, which will provide you with the link to the specific calendar you want to publish on the website.