The installer is surely not faulty, because it correctly puts the Esellerateengine.dll in the correct folder for your OS, which is C:\Windows if you run Windows XP, or C:\ProgramData\Esellerate if you run Vista or Windows 7.
However, on some systems, if you changed the default permissions or run in XP compatibility under Vista/Win7, it's possible the correct locations don't work anymore, in this case you can try the following:
- Be sure you don't run FS9 into "XP compatibility mode" under Vista/Win7
- Try running FS9 as Administrator, just to register
- If all fails, manually copy the C:\ProgramData\Esellerate\Esellerateengine.dll into the FS9 root folder.