If vehicles are missing, the program will stop or crash with errors. If they are invisible, but the program appear to work, it's nothing related to its installation or missing files, but more like a problem with your video settings or FSX settings.
Try the following:
- Reset the FSX.CFG to default settings, by removing it or moving to a different folder. File is located at %APPDATA%\Microsoft\FSX
- Reset your video card to default settings.
- Be sure the "Advanced animations" option is enabled in the FSX Graphic settings.
In any case, try also to download the full GSX installer now and install it, this will ensure you have all files in their correct place.