As you note, it's an add-on texture. You need the gown, or it won't work. The gown is for sale at RDNA.

If you have the gown, and still can't get to work, it's probably because this item is zipped differently than most. When you open it, you'll see another zip inside the zip. (Runtime.zip.) Unzip this second zip to your Poser folder, and you should see MAT files for applying the textures to the gown in a folder called PH-gown.