There's a sticky thread on the Knife Edge forums on how to download and import--I'm just a newbie, so if my instructions don't work for you you may want to poke around there instead...

For me it worked like this:

--In C:\Program Files\RealFlightG3\, create a new folder called something like "G3x downloads".

--Go to the appropriate post on the G3.5 aircraft swap thread; for the trainer version of the H9 P51, it is:
http://www.knifeedge.com/forums/down...o=file&id=2218

--Click on the file link "PTS-51-1- Trainer EA"; save the file to the folder you created.

--Launch RealFlight, and from the menu bar click on Simulation, roll down to Import, and then highlight and click on G3X.

--This will launch an "Open" browse window. Browse to the new folder you created, select the file named "PTS-51-1 Trainer_EA.g3x", and click the "Open" button. At the bottom of the RF simulation window, you'll see text that says something like "H9 P51 successfully imported."

To fly it:

--From the RF menu bar, click on Aircraft, roll down to and click on Select Aircraft
--This will open a "Select Aircraft" window. On the left-hand side is a file hierarchy; at the top should be a folder called "Custom Aircraft".
--Click on the "+" to open up that folder, and you should see the H9 P51 listed.
--Click on H9 P51; the first time, you'll see an alert that says "Creating DDS". It's generating the 3-D model.
--Once complete, click "OK".

Again, I'm a noob, so if this doesn't work for you please do spend some time in the KnifeEdge forum where I'm sure better instructions already exist.

Good luck and happy flying!