Once you have tested your popup and it is ready to display on you site, you need to activate it. If you do not activate a popup, it will not display to your visitors.
Activating a popup is really easy with Nelio Popups. Go to Nelio Popups » All Popups and there you will find a list with all your popups. For each popup, you have a toggle to activate and deactivate the popup in the Active column.

Once your popup is active, it will be available to be shown to your visitors. Therefore, make sure the popup is ready before activating it.
Additionally, when editing a popup, in the editor sidebar go to the Status panel. There you will find the activation toggle for the popup you are editing.

Therefore, you can activate your popup directly through the list of popups or within the popup editor.