When you want to add room you choose the accomdation where the new are and click on "Add a new room".
You give the room a name. Choose weather the accommodation are for rent or not. Chose how many rooms you want to add.
Choose the min. days in advance it has to be booked. Choose the min. amount of the the booking should be. Choose the the min. amount of time for cancelation.
Click on "Add"
The best overview in the calender comes when you only add 1 room and that every room are by them self.