I actually just found it last night myself after stumbling upon it.
You have to find a rock to push with Strength, which reveals a whole other area of the route, and in that area is a stairway which takes you into an alternate area of the Lost Hotel (different entrance); go inside and you will find a few trainers as well as the Dread Plate through a hallway.
I can't remember the exact route number on the top of my head, but you can just check one of the routes that are close to the Lost Hotel.
It's right next to a route entrance to the south of it coming out left if that makes any sense to you.
I will edit this answer with exact directions when I get a chance to play again.
- Edit: Late edit, but just in case you're still having trouble or for other people that don't know, it's in the West exit of Dendemille Town in Route 15, going passed the rock with Strength will lead you to Route 16. You will see the entrance to Lost Hotel in grass.