If you haven't reinstalled Windows, but simply installed FSX, with our sceneries is quite simple: you just download the FSX version of the installer from this site, and install under FSX. If the scenery was already activated under FS9, it will also run as Active in FSX too.
If, instead, you reinstalled Windows or want to use a different computer for FSX, install the FSX version of the scenery, and activate it using the "Register Serial" function of the Addon Manager, using the Serial Number as shown in your purchase receipt.