For G4:

Go to the "My Documents" folder and find a folder called "RealFlight G4".

Double click on that, then open the "Vehicles" folder.  In there you will find a folder named "CustomModels" open it, Inside you will find the "Blade_400_3D" folder open that.  Now download this zip file from here and extract the files to your desktop.  Place the 2 files in the "Blade_400_3D" folder and start up RealFlight, When you load the Blade 400 3D The error should be gone now.

For G5:

Go to the "My Documents" folder and find a folder called "RealFlight G5".

Double click on that, then open the "Vehicles" folder.  In there you will find a folder named "CustomModels" open it, Inside you will find the "Blade_400_3D" folder open that.  Now download this zip file from here and extract the files to your desktop.  Place the 2 files in the "Blade_400_3D" folder and start up RealFlight, When you load the Blade 400 3D The error should be gone now.

This same process also works under Vista, and Windows 7 as well.