It’s not exactly clear how the game will work, but asking ‘What’s missing?’ is natural. We’d ask, ‘What’s been added?’ though. ‘What’s added?’ would be a question to ask about something that is added habitually or all the time, not at the present time. You could also ask ‘What’s here that usually isn’t?’ Or you could say to them, ‘I’m going to add something to the room, and I want you to tell me what it was/is that I added.’ (Strangely both tenses of ‘to be’ are natural here) Also you can’t ‘tell’ questions. You have to ‘ask’ them.