Welcome to SST.
First please confirm that you are in the correct room, meaning that the room code to the right of your PET matches the stored room code in your PET that Marsinta gave you. The lift number, floor number and room number must all match.
If they match, all you need do is to select an item of furniture
by its icon in your PET and then press the PET's switch alongside of it. This will either deploy or stow the selected item.
As you will soon discover, what you choose to use, to not use, and the order in which you select them is fairly important.
This link may help you in that task.
Feel free to ask more questions, as you need!