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?

Layering is the key. Items on top of the scenery library list are drawn last.

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.

Yes, the scenery detail slider does influence what is drawn on screen. If your system can handle it (and any halfway up to date one can), it's best to just set it to maximum and forget about it.