[Solution]
I had the same problem running Vista 64. For some reason, Greystone installs, but isn't completely added into FSX's Scenery Library. This can be rectified by adding the scenery to the Scenery Library from within FSX.
1. Launch FSX
2. Go to "Settings"
3. On the right side menu, you will see something titled "Other Settings"
4. Click on "Scenery Library"
5. On the right side menu, select "Add Area..."
6. This should open your main FSX file folder, select the folder that says "fsdreamteam" and DOUBLE CLICK IT to open the folder.
7. Click on "GreystoneX" to highlight the folder, and then select "Okay."
8. This will add the total file to your Scenery Library. Click on "Okay" in the bottom right to close the settings menu.
9. No need to restart (this is a huge improvement over FS9), just click on "Free Flight" and you are ready to go.
- Anthony