I have downloaded a senery file W120n30.tgz  (southwest USA) from the world map into Flightgear/data/scenery.   

I have also downloded IZArc extractor program.

How do I get the extractor program to extract the files from the W120n30.tgz  file into the two sub folders of the scenery folder ('Objects' and 'Terrain') (just like flightgear did with W130n30 (san franscisco area) when I first installed flightgear)??