Hi Zac,
There are a couple of ways in which you can download & add aircraft to your FSX library. For me personally, I just go into google & type into the search bar what aircraft im after ie Saab 340 FSX downloads. Once you download a file, you will normally have to unzip the folder. I then normally cut & paste the downloaded aircraft folder into the FSX aircraft folder (ie C:\Program Files\Microsoft Games\Microsoft Flight Simulator X\SimObjects\Airplanes). This works for me so I'm hoping it will work for you. If you can't understand what i wrote above, don't hesitate to PM me
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Cheers,
Jayden