Hi bonnie. I'm sorry -- the link isn't correct. (Most likely I posted it incorrectly -- but Apple has recently made many changes to the User Manual which may have resulted in unexpected links.)
Try this link instead
It should take you to the sub-section in the User Manual at:
User Manual→Creating Slideshow Presentations→Modifying Settings Applied to the Entire Slideshow→Adding Text to Slides
If the link doesn't work correctly, simply navigate to that sub-section.
You can also search the User Manual for "add text to slideshow". That should bring up this page of results
There are two kinds of Slideshows in Aperture: ones based on Slideshow Presets, and ones based on Slideshow Albums. You must use a Slideshow Album in order to add text to slides.