The first requirement of course is to purchase FsPassengersP3D :)
http://www.fspassengers.com/?action=purchase

If you have already purchased and have your unlock code:

- Launch Prepar3D
- Press the 'ALT' key if you don't have the menu at the top
- In the FsPassengers menu select 'purchase and unlock FsPassengersP3D'






My Code doesn't work?

Pay attention to the fact that you must enter the unlock code EXACTLY as shown in the mail/webpage.
To avoid error you may switch Prepar3D into Windowed Mode (ALT+ENTER) and copy/paste
your unlock code into the unlock dialog.

See also:
Can I get a rebate if I upgrade from FsPassengersX or FsPassengers 2004?
Is money tight for you? I can make an exceptional rebate