How to reach Plane of Nightmare
The object you need to click to zone in to Plane of Nightmare is found in the 'yard' area of the building with Quellious closest to the sea. You have to enter this area from the south and make a few turns to reach PoN's zone in but it isn't really all that difficult to find.
Whatever it is you have to click is shown in the picture above. I'm not really sure what to call it, it kind of looks like a model building. Anyway, if you're still having trouble finding this model house thing take a gander at my map below for some extra guidance!