In Communications panel, select Manage EventStream (Actions menu on the right - the icon is a paper plane inside a speech bubble)
Under the Settings tab, there are three options: Leader Boards, Automated Posts, and Moderate.
Under “Leader Boards”, select which items you want to appear, and tailor the headings if you want to.
Since Leader Boards are a kind of automated post, Leader Boards will only work if the dates for Automated Posts are set up properly - the start and end dates must be no more than 1 day outside of your event dates.