Once you have logged into the system as an Administrator, in the upper right you'll see the Admin Button with your username. In the drop-list, click on Settings > Website Settings, and in the first tab that comes up (Calendar & Classes), scroll down to the second section and you can check the box for Add to Waitlist in the area for Class settings. Be sure to click Save at the bottom if you made any changes.
Once you have enabled that option, in each class in the Pricing tab you will have the ability to activate a Wait List for the class. Any students that add their names to the Wait List for classes that are fully booked will appear under Reports > Wait Lists.