1. Install PFPX v1.15 in a folder of your choice
2. Replace PFPX.EXE with the one from the \crack subdirectory
3. Start PFPX.EXE and wait for activation window
4. Check "Offline Activation"
5. Enter any serial number XXXXX-XXXXX-XXXXX (numbers only) and click continue
6. Enter your Name. This name will appear as registred user in PFPX
7. Enter any activation code. Again only numbers. Click ok and wait for thankyou message

Note: You have no official server subscription, so you won't be able to access the aerosoft online services. 
Also uncheck "Automatically check for updates" since these won't work with the cracked PFPX.