The X's are probably missing textures for draped polygons .pol (missing orthophotos, ground textures, etc) sometimes the texture is there but the pol needs to be placed again. Is hard to tell without seeing what the problem is. I sometimes use Overlay Editor to identify the trouble elements. And to add concrete there are several ways... the easiest is to add a taxiway with the taxiway tool. As I mention before, almost never the conversion is a one click deal, but if you spend some time correcting imperfections you can make it work really nice.