There should be a chamber hidden in the entrance to the Ruins. Serebii was unclear on this, and I've had difficulties with it as well.
You have to unlock all the rooms, where you need flash / Escape rope / Water Stone / Ho-oh and then when you have them click on the tabs behind those walls, and talk to the researcher guy, walk back into the Ruins of Alpha room. At the back theres a gold plate, examine it and he'll say hes studying it, you walk back with him. After that walk back into that small room with the stairs going down and you should find the ! , and using the mysterious sound you'll find the ?.
