If you want to allow a user to start and stop an automated slideshow, set showAutoPlayButton="TRUE" (in JuiceboxBuilder-Pro's 'Customize -> Button Bar' section) and the AutoPlay button will be displayed in the gallery's Button Bar.

If you want the slide show to start as soon as the gallery is loaded, set autoPlayOnLoad="TRUE" ('Customize -> AutoPlay').

If you want to have the images loop (instead of the slide show stopping at the last image), set enableLooping="TRUE" ('Customize -> General').

You can change the image transition type and image transition time using the imageTransitionType and imageTransitionTime configuration options ('Customize -> Main Image').

Other AutoPlay options (such as displayTime) can be found in the 'Customize -> AutoPlay' section.
Please see here for a list of AutoPlay options.