The current room name is displayed on the middle left part of the screen. Locate the small downward pointing arrow just to the right of the room name. Click on that Arrow.
The names of the existing rooms will be displayed in a drop down list. Click on the name of the room in which you want to place cabinets.
The room you have chosen will appear at the top of the list and that room will now be displayed on the screen. You can begin placing or changing cabinets in that room.