How do you install FlightGear Flight Simulator?

I was looking for a good flight simulator to use on my computer for free and found flightgear. Several people said they liked it online so I decided to install it. But after it finished downloading, it came up with a screen that said "FlightGear Launch Control". Under that it had places to fill in information like Executable and FG_Root. I have no idea what this means and what to put in these boxes. It won't let me continue until I've put something in there and the website has nothing on this. Could you tell me what this is for and what to put in there?