If you created a presentation in EW, edit that presentation and go to Inspector in the top right corner. Then click on Slide. From there, you can set the advancement of each slide. If all of your slides are using the same theme and you want them all to transition after the same amount of time, then you can set the timing on one of them and then click Apply to Theme at the bottom.

For instance, the image below shows this slide auto advancing after 5 seconds. I can click Apply to Theme at the bottom to apply that same setting to all slides using that same theme.

Don't click Loop Slide as this will just loop that single slide.

Also with the inspector open, you can click on each slide and check to make sure it's using that setting.