Unlike all other object types (picture frames, text frames, etc.), placeholders cannot be inserted directly into a slide.
Placeholders are provided only via Presentations' predefined slide layouts that you can apply to a slide. Each slide layout contains a different number and arrangement of placeholders.
To change the layout of a slide, choose the ribbon command Design | group Design | Layout 
and select the desired layout.
For more information, see Changing the slide layout.