You can find this countdown in the Feedback page. This clock shows the time left until your next retrospective meeting.
Once, the countdown reaches 0 this means your retrospective is ready for you and a Start Meeting Presentation button will appear.
This button will be available for the next 6 hours and after that time your next retrospective scheduled will start showing.
In order to access the Presentation View just click on View presentation and the agenda for your retrospective will be shown.