Righto. I'll assume you know how to install an aircraft to a previous version of FS.
I'm using the method Felix described earlier but you could install the aircraft to the FSX\SimObjects\airplanes folder if you wish. First you need to install the original FS9 aircraft, Mustang_P51D.zip. If you open Sound.cfg in Notepad you will see that it's aliased to the FS9 default DeHavilland_Comet. The aircraft should display on the selection menu but without any sound.
Next the update, xP51Dup.zip. The best way to install this would be to extract the zipfile to a temp folder & paste the contents into the original aircraft in FSX. Overwrite files/folders when prompted. I noticed there is no updated Sound folder or Sound.cfg so unless you have the FS9 Comet in the same folder as the Mustang there will still be no sound. This can easily be fixed by re-aliasing the sound to an aircraft you do have. Better still would be to find a nice Merlin sound package & install that.
Here's the Mustang in the FSX Demo.
PS. A couple of gauges are included with the update. You can paste them into the Mustang Panel folder if you wish but if FSX is like previous versions of FS this shouldn't be necessary. Please be careful overwriting gauges in the main FSX\Gauges folder. If in doubt don't take the chance. I hope this is some help.
PPS. I forgot to mention that the original FS9 version of this aircraft is a zipfile within a zipfile. I don't know why it was configured in this way but make sure you extract the second zipfile & then install the correct folder to FSX\Aircraft or it won't work. The folder name is Mustang P51D. This directly contains the familiar Model, Panel, Sound & Texture subfolders.