Blender Surface Roughness I'm trying to model a cube with a rough surface on only one side of the cube, while keeping the other sides relatively smooth. I've been looking around, and so far, all I can find is subdividing and then the displacement modifier. I'm pretty new to Blender, and I'm confused as to how I can apply a rough surface ( not just textured with an image - I'd like the cube to physically have those bumps ) to the cube? I can only find tutorials for working with a plane and making it rough via subdivisions. Any help would be greatly appreciated!! 

EDIT: @Carlo, yes it's because I'm unable to pick on of the six faces. Basically what I'm looking to do is to create only one surface of the cube rough, while every other side remains smooth. Does this clarify my question a bit? Sorry about the delay in response!
Solution is simple:


You go in edit mode and select desired face.
Click W, than select Subdivide.
Either repeat previous step multiple times or on the bottom left part of screen you will see a value Number of Cuts that you can adjust to your needs.
Select the subdividen faces and in Mesh tab add a new vertex grop and assign those objects to it.
Add displacement modifier, select the verex group under it's setting, select texture and adjust values.


Done! ;)