This file ONLY works with FS2002. It defines taxi routes, parking and other facilities on the airport you've downloaded.
You must just put into any old folder - even on your desktop if you like - but recommend that you create an AFCAD folder within your main FS2002 folder if you are going to download more sceneries in the future.
Then you must download and install AFCAD 1.4, start the program and click on File..Import Airport. Then browse to point to this file, wherever you've stored it.
That's all. It'll set the airport up in the way the designer intended.
On the other hand, if you're running 2004, the above is all balls