As far as I can tell, this first (1.0) version of the Photos application does not support displaying any of the attributes of the pictures in a slideshow, not even the "title" text. It supports adding a text box on top of a slide in a fixed location, and will auto-populate a text box bearing the slideshow name on the first slide. After that, you have to add the text box (click "+" in a circle, select "Add text") and edit the default text (which is literally "Default Text").
If you want Apple to add a feature, either one that was in iPhoto or a new one) to the Photos application, tell them through the official Photos feedback page:http://www.apple.com/feedback/photos.html
They almost never respond, but they will have the opportunity to consider the comments in future development.