To install the Nature Terrain Generator plugin for iClone 8, there are separate installers.
You can visit and log into this page:
https://www.reallusion.com/member/#/en/order-history
Of you can find your Nature Terrain Generator plugin and download the installer for iClone 8.
(Note: If you have trouble installing, then kindly uninstall the Nature Terrain Generator plugin with the Windows Uninstaller first.)
After installing, download the content pack with the Content Manager for iClone 8.