new materials on imported 3d objects

[sutcac]

I apply custom materials to custom symbols all the time.

Which format was the symbol you imported in? Did you check "Double Sided" for the faces on import? (You can also change this after the fact by using Open Symbol).

It may be that your faces are reversed and that the material is being applied to the "inside" of the object where you can't see it. I have run into this occasionally.

You can reverse these faces in Sketchup and, I believe, Accutrans.