Well, there's a little more to your request than just "get Gmax".

First off, Gmax isn't exactly the program you make the whole airport with.  For that, I would recomend FSSC (Flight Simulator Scenery Creator).  It will aid you in placing the runways, autogen buildings, etc.

Then there is AFCAD 2.  It will edit the new airport for landing AI aircraft and other little scenery tweeks.  Use it after you get the airport built.

Now, if there is a building that you need to create from scratch (like the unique ones at LAX or something) then you might want Gmax.  This is the easiest one to get.  Check the fourth disk of the FS2004 set.  It has the setup utility on it, and you can get the registration utility (for registering without going through a bunch of hassle ) HERE.  Use Gmax only if you are creating extra buildings, as the other two programs can do the rest.  Make sure you also get the Stupid Idiot's Guide to Gmax on that same page.  It will guide you through creating Gmax stuff.

And as always, DON'T GET DISCOURAGED!!  Many people do, and never finish their projects.  If you have questions, just come on back and ask them.  We'll all be happy to give pointers.

Good Designing!

Locke