It works on most (if not all) aircraft that already have a HUD. The "polygon" that the original HUD is displayed on, is also used for the Weapon for FSX HUD.

You remove the original 3D HUD (fa-18!MFD) and replace it with the Weapon for FSX HUD (CSWeapon!hud3d).
The example I used above is the CS_F/A-18, which uses the default Acceleration FA-18 VC and therefore it will use the HUD "poly" that comes with it.

Hope this makes sense.