You can find this countdown in the Agenda page. This clock shows the time left until your retrospective meeting starts.
Once, the countdown reaches 0 this means your retrospective is ready for you and a Start Meeting 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 page just click on Start Meeting button and the agenda for your retrospective will be shown.