Thank you Bjoern, No it wasn`t that. I had to adjust the Scenery Display Settings from default to higher detail. Then the add ons showed up as they should do. So that is something I have learned. Do not install add ons until you have altered the default Scenery Display Settings.
I am having difficulty in installing add on scenery on a friends computer. I have successfully installed New Zealand Vector Land Class but when I install other add ons although they appear in the Add on Scenery folder and I activate them in the scenery library they do not overwrite the default scenery. I am quite proficient at installing and activating add on sceneries and I cannot understand why they are not showing up in my friends FSX program which was correctly installed with both service packs an is running fine. Any ideas please?