The SERVER CONTROLS will open in a panel to the right:
Click the same “double arrow” button to connect to the main server:
Here, I am in the “MAIN ROOM”.
From here double click on the room you wish to join, or type a name into "New Game" to create a new room. If you are starting a new room, from there click on File, and then "Let's Play." If you are joining a game that is in progress, right click on one of the participants and select "Synchronize."