There are two ways to start a slide show. Both ways are available when you are viewing a list of photos.
The Action Bar at the top of the screen contains a "Slide Show" button. When you click on the button, it will start a slide show using the current list of photos (up to 100 photos).
If you want to view a slide show of photos you choose, you can right click on a photo you have selected and then choose "Slide Show" from the popup context menu.
While slide show is in progress, you can play background music if desired. Click on any part of the screen will toggle the display of slide show controls. You can use the music control to start and stop playing background music. Note that the music is selected randomly from your music collection. Currently it's not possible to choose specific music for slide show.