To add an additional calendar to your site:
Create a new folder & give it your new calendar's title.
In the green edit bar, select calendar from the sub-types menu:
Note: If a folder is already set to display as a calendar, a bullet point will display in front of the calendar option in the sub-types menu (right).
To enable adding events to your calendar, you must change the content type restrictions within this folder. Select restrictions... from the add new... menu:
On the Restrict what types of content can be added page, select the checkbox beside Event, then save:
You should now be able to add events to your new calendar from the add new... menu within this folder: