In order to join or create a presentation room, you will have to first click on the navigation button, which is the second one from the sidebar to the left of the screen.
By clicking it, you will reveal the navigation menu:
Click in Meeting Rooms, this will bring up some options according to room types.
You can join a public room or create a private one for yourself and your team.
Join a Public Meeting Room
If you want to join a public meeting room, you just need to click in the room type of your choice and click on 'Create/Join' and then 'Yup'
Join a Private Meeting Room
If you want to create your own private Presentation Room, you can do so by selecting the proper option when creating one, entering a safe passcode like this: