The instructions for having a pre-order says: 

"A project is a pre-order if all its public uploads have the This file is a pre-order placeholder checkbox ticked. It will allow buyers to purchase before uploads are made available."

I don't see that checkbox. Where is it or what is going on?

Okay, I figured it out. After you change the settings for a project to charge money, you can start using placeholders for free demos and pre-orders.