Select Virtual Tours -> Rooms from the side menu and click the Edit button on the desired room.
Go to tab Content / Annotation and add annotations which will be visible in the room.
Click Save on top of the page.
You can adjust the annotations settings inside the Editor UI, like the position and style.
Select Virtual Tours -> Editor UI from the side menu. Click on the Annotation box.
Click Save on top right corner of the page.
This is an example of the end result.