If you select one cube face at a time, and apply the six different textures, it should eventually work. Are your images square to start with?
The "Flat" wrap mode has to be selected, then those two buttons are active.
I had to fiddle with the "Normal" settings for each face, changing from X- etc to get them to show. One has to click those "fit to boundary" buttons for each face. But I did get a cube with 6 different images.