I believe that differential braking is built into the model in the case of the P-51.
However, you can add the feature to any aircraft in your hangar by editing the aircraft.cfg file in 'Notepad'. Although it is generally only used on 'tail draggers' it can be useful on multi-engined aircraft if you only have a single throttle to control all engines and therefore cannot easily make 'engine turns'.
To add the feature open up your aircraft.cfg file of choice in 'Notepad' and scroll through until you find the [brakes] section and then add the following line:
differential_braking_scale = 1.0
Save the file after adding the line and you should be good to go.