Consult the theme description again please where you will find instructions for "A Dramatic Personalized Home Page". http://theme.wordpress.com/themes/forever/ Live demo site. http://foreverdemo.wordpress.com/
You need sticky posts with featured images for the slider to display.
This is the guide for setting a featured image on any theme http://en.support.wordpress.com/featured-images/#setting-a-featured-image
If you don’t see the Featured Image module on your Add New or Edit Post page, please make sure you’ve selected Featured Image in your Screen Options. http://en.support.wordpress.com/screen-options
Also note to show only your four latest posts (with a Featured Image above each one, of course) in a neat grid on your home page visit Appearance → Theme Options and click the checkbox there.
Thumbnail on 4-column showcase for recent posts. Width 195px, maximum height 124px (cropped if larger).
Featured Image (at least 887 pixels wide and height 355px)