Using CiviCRM shortcode (wordpress) for a profile in a popup
I have had success with Popup Builder - the preview never works, but the embed on the page always works.  This is a popup overlay - which I assume is what you are looking for. The free version is great if you just want a popup on a page of your choosing - the paid version is great if you want the popup to come up on any page a first time visitor first sees.