Presenter has no method of adding captions to a presentation. The best option is to put the text in the notes section of PPT, thus having the text show in the Notes pane in the side bar. Alternatively, you could put text on the PPT slide and use entrance/exit animations to have that text appear and disappear when relevant.
One of the Presenter 8 webinars mentions a feature to add captions -
"Become the Superstar of your University’s Online Course Catalog" available at
at http://www.adobe.com/cfusion/event/index.cfm?event=set_registered&id=2180510&loc=en_us. Is that some extension that has to be installed.