Invisible objects are usually caused by a problem with video settings. Try all the following:
- Update your video driver
- Reset your video driver settings to default
- Reset the FSX settings to default, by removing the following file:
%APPDATA%\Microsoft\FSX\FSX.CFG
This will force FSX to rebuild a new config with all default settings. After doing this, be sure you apply the HIGHMEMFIX tweak in the Addon Manager, save the settings and restart FSX.