The Add Area function under Scenery Library somehow fails to install new sceneries under Windows 7.   The simple solution is to edit the Scenery.CFG file manually in order to place new addons to the Scenery Library as described below:

1. Locate Scenery.CFG file from  C:\ProgramData\Microsoft\FSX\
2. Make a backup of the Scenery.CFG file before making any changes.
3. Open the file with Notepad and go to the end of it.
4. Copy the last scenery section and Paste to the end.
5. Add 1 to the number of both Area and Layer.
6. Change Title as required
7. Change Local as required  (directory of the new scenery)

[Area.Last number+1]
Title=Name of the New Scenery
Local=Directory of the New Scenery
Layer=Last number+1
Active=TRUE
Required=FALSE

8. Saved and exit.
9. Run FSX and move the New Scenery, if required, to the correct position inside Scenery Library (or use the FSTscenery tool from Steve Greenwood.)