Once you have made your final selection, click on the Apply to All Slides button.
If you want to apply this slide transition to only specific slides, select those slides in the Outline / Slides pane first, before choosing the transition.
From Wendy Russell, former About.com Guide
Once you have made your final selection, click on the Apply to All Slides button.
If you want to apply this slide transition to only specific slides, select those slides in the Outline / Slides pane first, before choosing the transition.