This shared calendar works in a strange way. When you enter a calendar item, it is tied to a posting.
To see the Calendar feature, please click on the Calendar link right at the top menu bar of the website. Then, you have to identify which section of the forum that you want to post in. I have created a forum section called Calendar Events right at the bottom of the screen. Please see the attachment.
Once you have identified this, you can then post the title of the calendar entry which will then appear in the calendar.
Certainly a strange way of entering a calendar item but this is how this system is designed!