By the right of the room title, click the pencil icon to access the editing page of the plan.
On the page, click add a table on the left, and choose the type of table you are adding to the room. Place it on the plan, then, name the table, enter the number of covers and rotate it if needed. You can also duplicate it or remove it by clicking the bin.
Click confirm in the left, then, on the right, click register to save the table.