Correct me if i'm wrong, but i think you're talking about missing textures, not materials, right? It's most likely paths issue, for one reason or another, 3ds Max can't find the files at the specified locations. I don't know how Corona material library file paths are organized, since i don't use it, but you need to find a way how to resolve those paths, otherwise the textures will be missing. Same thing applies to Forest Pack's missing assets.