To add a virtual wall in a room

Click the Add Wall icon.
Select the room you want to divide with a virtual wall.
The tool becomes active. A dynamically updating line is attached to the pointer, and to the boundaries of the room. Positioning the pointer closer to one side of the room changes the dynamics attaching the line to the closest room boundary.
Data point to enter the first point of the virtual wall.
Continue entering data points inside the room to create virtual wall segments as needed.
The final data point finishes the virtual wall. That point is any point on the any of the room's physical walls.
The virtual wall is completed and added to the OpenBuildings Energy Simulator Project Tree as a Room Division object.
Do not reset yet. Rather, continue by selecting another room, and repeating the previous steps, create another virtual wall there.
Reset to completely exit the Add Wall tool.