You must be an Account Manager to create a new event site. If you aren't sure - check the Team List on an Event Site Dashboard to see all Account Managers plus the Chairs/Admins/Volunteer/Staff for that particular event.
To Create a New Event Site:
From the Dashboard, click on Event Sites > Manage:
Choose the URL for your new event site
On this screen, you'll provide basic information about your event. Once the event site is created, the URL is NOT editable - so choose carefully. All other fields (Name, Date/Time, Timezone) can be edited later from your Account page or the event site site settings.