To Install into FS2004: 

1) Extract the zip & you will find an 'Aircraft' folder.
2) Everything in this 'Aircraft' folder needs to go into your 'Flight Simulator 9/Aircraft' folder exactly as it is.
3) If there is also an 'Effects' folder, everything in this folder goes in your 'Flight Simulator 9/Effects' folder.
4) If there is also a 'Texture' folder, everything in this folder goes in your 'Flight Simulator 9/Texture' folder. 
5) There is no number five.



FSX Install
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This is an FS2004 model, but it does work in FSX with a few minor faults.
The main faults are :-
black landing gear
Kneeboard can no longer view multiple pages
Built in VC view controls do not work correctly

Install in FSX is different because they have changed the folder structure.
\Aircraft = \SimObjects\airplanes

Extra sounds for co-pilot calls & takeoff config warning need files editing because they've changed the folder structure.
The panel.cfg in '\DMFS Shared Files' needs this line modified...
gauge32=dsd_xml_sound3!dsd_xml_sound3,  2,2,5,5,.\Aircraft\DMFS Shared Files\DH106 Comet 4 Panel VC\DM_COMET_sound.ini
change it to be...
gauge32=dsd_xml_sound3!dsd_xml_sound3,  2,2,5,5,.\SimObjects\airplanes\DMFS Shared Files\DH106 Comet 4 Panel VC\DM_COMET_sound.ini

The file DM_COMET_sound.ini in '\DMFS Shared Files' also needs the paths editing.
For example...
Sound00=./Aircraft/DMFS Shared Files/DH106 Comet 4 Panel VC/ALERT.wav
Will need to be...
Sound00=./SimObjects/airplanes/DMFS Shared Files/DH106 Comet 4 Panel VC/ALERT.wav


The smoke effect also needs a file edit to work properly in FSX.
Each aircraft.cfg file has a section called [SMOKESYSTEM]
Remove the first 4 entries, then edit out the '//' from the last 4 entires.
The end result should look something like this...
[SMOKESYSTEM]
Smoke.0= -9.9, -13.0, 0.00, DM_COMET_SMOKE_FSX_R
Smoke.1= -9.9, -11.0, 0.00, DM_COMET_SMOKE_FSX_R
Smoke.2= -9.9, 9.0, 0.00, DM_COMET_SMOKE_FSX_L
Smoke.3= -9.9, 11.0, 0.00, DM_COMET_SMOKE_FSX_L

Thanks to Ben Watson for the FSX smoke effect.