how to order the photos in a slideshow

Just like for any other photo - select the photos in the album and use the command "File > Order Prints".

Be careful, not to include any text slides. If it is difficult to select the photos in the slideshow, create a new album from the photos in the film strip (File > New Album).

Super fast resolution per Tiffany of Apple Support (Thanks, Tiffany!) - it's a bug in the Slideshow Button featured at the bottom of Albums and Events windows. You will need to create an actual slideshow, as follows:

Select the images you want to use --> chose "Add To" --> "Slideshow"--> "New" or chose an existing slideshow.

You will be able to manipulate your slideshow options (theme, music, order, etc) from there.

I don't see amy slide show button on the bottom of my Photo album. It all comes out of the mouse button. I cannot arrange anything. I can keep the order they have and nothing else.