Bookings made with your My Eurotunnel app or account can take a few minutes for it to be displayed in the app. Usually by the time you receive your booking confirmation email it should be visible.
If it's not visible, on the home screen or booking tab, pull-down-to-refresh to retrieve the latest data, or force-close the app and re-open it.
Guest booking (booking not made with your account) will not be available in the app, and cannot be added to it.