Go to the SLIDE SHOW tab... and "Set Up Slide Show".
Then set it to "Browsed by individual" (this will automatically set to advance using the timings you've added).
Go to Slide Sorter view for your slide show. Either click on the button at the bottom right of your window...
... or go to the VIEW tab and click on the button there.
Go to the TRANSITIONS tab. With the first slide selected, add a transition using the drop-down menu (you'll see lots
Add a variety of transition types to each slide.
Set them all to advance after 15 seconds (far right, not under Duration!)
Click off the slides so none are selected.
Select just the first one and change it to "5". When you [Enter], it'll change the first one to be only 5 seconds.