Click the + icon to add a stack to your page (turns into pencil icon after clicking) - the User Comment Area stack item can be found under the Tools section.
To set up your User Comment Area, you can click the pencil icon to edit the stack.
You can set up whether comments must be approved, and who can approve them.
You can also use the start and end dates to schedule the comment area for a set amount of time.
Don't forget to save your changes before going back to site!