Also , if you just want a yellow box and a black box , do this.

1. import your box object.
   2.go to material room , and click on the Diffuse colr, set it yellow
2. Back to pose room
3. select props library, click the + sign at bottom
   Name your prop Yellow_box and save it
4. back to material room, set diffuse color black
   etc etc etc
5. Save to props library as Black_box

done

You dont need an image map for textures if you just want a plain solid color.