Hello, in case anyone tries to install this plane and has trouble, I have found this link to work fine:
http://wiki.flightgear.org/North_American_P-51_MustangClick on the little folder icon near where it says download on the right of the page.
For it to work right, you need to make a second folder called p51d that has contains the real p51d folder with all the model info:
In Flightgear Addon Menu it should look something like this, you're going to need a Custom Aircraft folder if you don't have one.
I hope this helps!
EDIT: I suggest trying this method ONLY if the Launcher installation of this aircraft gives you errors.