It's not very clear from your message, if you are missing stairs in GSX (as the subject of your post leads to believe) or you are missing stairs ONLY with that 787 (as your post seems to say) or you haven't even tried with another airplane, so you don't know if it's "GSX missing stairs" or it's really "missing stairs with the Aerosim 787".
Since GSX surely has stairs, I can only assume either you mean that are you missing stairs ONLY with the Aerosim 787, or you haven't tried with another airplane.
With an unsupported airplane, you should first create a configuration file for it using the GSX airplane configuration editor because, if there isn't one, GSX will TRY to create a very rough configuration based on the aircraft.cfg, but that data is not very precise so, it's possible the door's height is not correct or it's not even there (some airplanes don't have doors with coordinates in their aircraft.cfg), which means if GSX doesn't know where the doors are because your aircraft.cfg it's either wrong or missing data, it can't place its vehicles.
Which is why, GSX has an integrated airplane configuration editor, that will allow you to add an unsupported airplane in a few minutes. The GSX manual has a complete explanation how to use it.
