How do I add additional photos to an Aperture Slideshow already created

(Sorry — mistakes corrected.)

The Slideshow itself is created using an Aperture container properly called a Slideshow Album (and unhelpfully referred to in the program and its User Manual as a "Slideshow").

I found it helpful to distinguish between these special-purpose containers (e.g.: Slideshow Album), the work-in-progress they contain (which is really a blueprint for creating a publishable file or set of files {no name given ... a "Slideshow Blueprint"?}), and the made item (the Slideshow as a shareable file).

Add Images to an existing Slideshow Album the same way you add Images to any Album. All Images in a Slideshow Album show in the Slideshow Blueprint. When you need a shareable Slideshow, create it by exporting. (Here, as elsewhere, "Export" means "Make me a shareable file of this object (these objects) using these settings I specify". Your workflow in Aperture is almost always: Store, Group, Make Better, Publish.)

There is a whole chapter in the (quite good for its time, actually) User Manual, available on-line and from the Help menu.

If you aren't secure in your use of Albums, here is a short intro to the parts of Aperture.

HTH.

—Kirby.