In VirtualBox you can select a VM from the list (it has to be shut down when you do that and not in a saved state) and then right click on it and click on Settings. In the pop-up that opens you choose Storage in left panel and then in the right panel under Storage Tree you click on the + icon next to the Controller IDE. You can browse for the ISO and then add it to the storage. After that you can start the VM and you will have access to the SS ISO like it was a CD drive.

Not all these steps are explained in the book, many details you will have to figure out by yourself.