As I say in my books, if you right-click on any transition in the Transitions panel, you will see the option to Set As Default.
Once it is set as a default, you can use the Project panel's Slideshow Creator to create a slideshow that uses that transition between each slide.
Also, note that there are actually four ways to build a slideshow in Premiere Elements. The Organizer's Slideshow Creator also includes the option to add any transition -- or even random transitions -- between your slides.
My books take you step-by-step using both those tools, if you're interested.
Thanks for that , I know it will do the job, could not find it in any Adobe manual, will have a look at your books.