Zaphod, it is disorganized, so just need to sort things out.

Extract the download to a temporary folder. Open the temp folder and browse to the \Ascension Island\ folder.

In the \Ascension Island\ folder, open the \Aircraft\ sub folder. Move the 3 AI aircraft folders to your FSX\Simobjects\Airplanes\ folder. (or wherever you install your addon or ai aircraft.)  If you don't intend to fly these 3 aircraft you can delete the \panel\ and \sound\ folders from each. This will keep the aircraft from showing in the FSX select aircraft window.

In the \Ascension Island\ folder, open the \Effects\ folder. Move the 2 .fx files and the \Texture\ subfolder to your FSX\Effects\ folder. When asked to 'Merge' the Texture folder, say yes.

In the \Ascension Island\ folder, open the \FSX Brize Norton\Scenery\ folder. Move the 2 .bgl files to the \Ascension Island\Scenery\ folder. (CAUTION: If you already have an addon scenery for Brize Norton, skip this step to avoid conflicts. If you ever install another Brize Norton addon scenery you need to remove these 2 files.)

In the \Ascension Island\ folder you can now delete the \Aircraft\ and \Effects\ and \FSX Brize Norton\ sub folders to clean things up.

The \Ascension Island\ folder should now have just 2 sub-folders: \Scenery\ and \Texture\.

Move the \Ascension Island\ folder to your FSX\Addon scenery\ folder (or wherever you install your addon scenery).

In FSX, add Ascension Island to the Scenery Library.

Dave