You can create a new room when adding a manual class. Simply select 'Add new room' from the drop down box and click the "create class" button, and you will be taken to the room creation screen.
You can also add rooms from the Rooms menu. This can be accessed by selecting 'Rooms' from the 3 lines menu in the top right hand corner. Once in the Rooms menu select 'Add Room'
Thirdly, you can create a new room from within a class. mouse-over the 'Rooms' tab at the top of the screen and select '+...add new room' from the drop down list.
After going through any of these channels, you will now be in the 'Add new room' screen. You can now name your room and choose whether to prefill the room with desks. Selecting to prefill the room with desks will allow you to add single or double desks in as many rows and columns as you wish.
You can now drag desks from the sidebar on the right hand side into the room to represent your classroom layout.
You have now finished creating a room. Now simply click any button along the top pane to leave this view. Your room saves automatically.