The Slytherin common room was a long, low underground room with rough stone walls and ceiling, from which round, greenish lamps were hanging on chains. A fire was crackling under an elaborately carved mantelpiece ahead of them.
As a non-Slytherin student, you have to make Polyjuice Potion in order to get inside Slytherin's Common Room. After you finish creating 2 parts of Polyjuice Potion, you can come back to this moment to click on the wall that will slide open. After that, you can zoom in and explore Slytherin's Common Room.