OK. Please send us all files located in
My Documents*\Prepar3D v4 Add-ons\Seattle X . There should be 2 files.
*I mean this folder:
https://en.wikipedia.org/wiki/My_Documents
To me this seems like broken registry entries. The installer tries to intall a file into the \Addon Scenery\Seattle X\Documents but there is no such folder. The correct path would be for example
D:\Prepar3D\Addon Scenery\Seattle X\Documents . There probably IS an entry in the registry but without any value and this obviously will confuse all installers. You can check that easily by yourself:
Press Windows+R, enter "regedit" and press Enter. Now navigate to the proper directory, which for P3Dv4 is:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Lockheed Martin\Prepar3D v4 - key name is "SetupPath"
or
HKEY_LOCAL_MACHINE\SOFTWARE\Lockheed Martin\Prepar3D v4 - key name is "SetupPath"
or
HKEY_CURRENT_USER\SOFTWARE\Lockheed Martin\Prepar3D v4 - key name is "AppPath"
Check the key value. It should point to where your P3Dv4 is installed but in your case it is probably empty. Additionally something (security settings or other programs) might be blocking the installers from reading that path, but I am almost sure about the solution above.