First of all you go to

admin > system > design > banners

and there you either edit or insert banners.

When you clicked insert (or edit) you will be taken to the banner page. In that page you have to insert more images by clicking “Add banner” if then you want a slideshow. After you uploaded your images in the banner (is best if the images are the same size) , navigate trough admin to

extension > modules.

There , click edit on the module Slideshow. There , you will click add module , and in the first column , its asking you to select a banner. So , there you select the banner in which you uploaded more that one image (the images from that banner will appear on your slideshow.

The second column in size. Is best to insert the same size for the slideshow , as for your images (if you want it like the one from the demo , the size in 980X280). At layout select first home , and then if you want it on other pages too , click on add module , and add it to another layout too. At position select content top (or some other position you want) , then select Enabled and hit save.
Please note that the pages (layouts) in which you use the 980px width banners. 480px height.