For polygon smoothing to work you hae to render polygons in back to front (or front to back, can’t remember) order. glutSolidCube won’t work. You also have to set the appropriate blend mode. There are about 100 threads on this at these forums (at least in the advanced forum), do a search and you will be enlightened.
I’ve already posted what you need to do to get the cube to render correctly, look at my post above. You can’t use glutSolidCube. What part of this are you confused about?