When you go into the room, you will see 5 different appliances on the floor. If you have Rotom with you, you can go up to any of them and select it. Once selected, Rotom will possess the appliance and thus you now have one of the new Rotom Forms. However there are a few things to note.
When you select the appliance and Rotom takes possession of it, Rotom will have the ability to learn a new attack. This is dependant on the Form that is is taking. This attack can only be kept upon this particular form. Once you change to another form, Rotom will forget that attack and learn a new one.