To set up reminders for calendar events you must turn on Notification in your User Settings.
Click My Settings from the drop-down menu beside your avatar, then scroll down to Notifications in the User Settings modal. Click on the boxes under Text, Email and Popup to turn On and Off.
To turn on a reminder per event:
Click on the Add a reminder link in the right section of a New Event modal.
Select whether you want to be notified via Text, Email or Popup and when you want to receive the reminder. For example, a Text notification will be sent 5 Minutes before the Event.