For the example above... enter the Texture room to edit the texture of your box.

A. On one side of your box, using the "Rectangle Layer" tool, click and drag a square shape.

B. On the shader tree under "Rectangle Layer" , select "Multi-Channel" for your shader. Then under that multi-channel, select "Texture Map" for your color and then load a texture map. You should see it appear on your box.

C. Then, still on the shader tree under "Rectangle Layer" for the Opacity Mask select "Texture Map" and load your mask layer.

Hope that helps
Chuck