According to the readme file for this product that is posted in the Marketplace, the package SHOULD have come with a file named Schooltheater.pz3.  The readme says that all you'd need to do after opening that scene is add your characters. There's also a file named school_main.pz3 but I'm not sure what that one is. unless it's the backstage basement mentioned in the readme.

A complete cinema-theater scene (including backstage basement) full of people that comes with all props and Raytrace lightning.
   A pack of all that you need in order to give amazing renders to your figures! No need to wary for 'plastic look' or washed out shadows. 
   All the props are there, most of them can fly around, all the lights are at the correct position and give realistic shadows. 
   They are organized in groups with the correct order so that you can quickly show and hide props to render from deferent angles. 
   For best results on shadows, use Raytrace rendering.

Since this product was made BEFORE Poser added a Scene tab in the library, it looks like it was placed in the runtime root folder.  So, if the root folder is (for example) Poser 13 Content, you'll find the PZ3 file in THAT folder, and not in Poser 13 Content > Runtime.

If you are having problems with a product, always check the readme first ;-)