Select the option "after visitor sees the content once only" (either in the main Countdown Dynamite admin screen, or in the widget below the content area on the edit-post or edit-page screen.)
Go to edit the post or page you want to have the OTO on and make sure you select the "Enable countdown on this page" option, from the Countdown Dynamite widget.
There's no need to insert any shorcodes in the post or page (since this will just be a one-time offer and no countdowns need to exist in the content).
Article ID: 105
Category: Countdown Dynamite Wordpress Plugin