If you've downloaded the Riddlez NZNP .exe installer fromhttp://www.riddlezinteractive.com/sim/nznp
And run the installer.
It will automatically create a Folder called Riddlez within the Main FSX directory.
You might need to check where the installer installed the Riddlez Folder.
Within the Riddlez folder you will find the following directory name NZ New Plymouth Airport NZNP 2016 and the uninstaller uninst-Rid-New_Plymouth_FSX.exe
Within the NZ New Plymouth Airport NZNP 2016 folder you will find the Scenery and Texture folders.
You can either choose to move the NZ New Plymouth Airport NZNP 2016 folder from the Riddlez folder and Move To the Addon Scenery folder and activated from here in the Add On scenery library with the the directory path
FSX\Addon Scenery\NZ New Plymouth Airport NZNP 2016\Scenery
Or go to the FSX Settings Scenery Library and Add Area from the directory path for the main FSX\Riddlez\NZ New Plymouth Airport NZNP 2016\Scenery
This works in much the same way as other developers such as Godzone, Orbx, VLC all have there Auto Installer Directories in the Main FSX folder outside the Add On Scenery Folder.
If you have the older Riddlez version Folder "NZ New Plymouth Airport Scenery 2013" in the the Add On Scenery folder you will need to either Remove or Disable this from the Add On Scenery or perhaps check to see if you have some other form of conflicting NZNP ????.bgl files possibly located in another scenery directory somewhere.