You will need to have Google Earth installed before you can actually access this function. Once Google Earth is installed properly, start it and then go to Tools > Enter Flight Simulator. Or you could directly press CTRL + Alt + A using your keyboard.
The box will open allowing you to customize the aircraft and the start position. If you have a Joystick, make sure to check this option to control everything from the joystick.
For additional information, access the Google Support Page: https://support.google.com/earth/answer/148089?hl=en
