Heya GunnerMan,
I agree with ReverseThrust that AFCAD 2.21 by Lee Swordy is best for editing basic airport parameters (Runway and Taxiway placement, alignment, and headings, adding helicopter landing pads, Gates, Ramps, Parking, & etc.).
If you need to place airport control towers, buildings, hangers, Gate structures, and boarding ramps into FS 2004 scenery this is a two-step process.
1.) First you have to remove the incorrectly placed buildings and structures from the default FS 2004 scenery. I use an exclude statement *.bgl to do this. If you don't know how to do this there are a lot of really nice helpful folks in the SimV Scenery Forum who can help you out.
2.) I use two programs (both payware) to insert items into FS 2004 Scenery. The first program is Lago's Flight Simulator Enhancer and is available here:
http://www.lagosim.com/default.asp?act=home&lang=engLook for this program under the title "Best Sellers" in the middle of the webpage.
This program is free to try, however, if you want to place more than 5 items into your scenery you need to purchase a registration key (like I did) for about $30.00 US. With a registration key you can place unlimited items into your FS 2004 Scenery.
There are many things that I like about this program. It's easy to use (you don't have to learn extensive amounts of code) and it comes with hundreds of scenery items to place into your FS 2004 world. You can make dynamic scenery items (Trucks that move on your airport taxiways, Buses that drive around, airport maintenance vehicles & etc.) and you can also program in sounds so that if your aircraft gets within a certain point you will hear the sound. As an example I used this program to place a hanger at my home airport (KDAL) where there is a hanger in real life. My FS 2004 starts out with my aircraft (with all systems, engines, & etc. switched off) sitting in front of this hanger. I also used Lago's FSE to place an aircraft towing vehicle, baggage car, and aircraft external power vehicle close by. In the background you can hear the sound of a diesel motor running (for the external power vehicle).
This program is very versatile and comes with complete instructions and help files. In my opinion it is well worth the cost.
The second program I have purchased (for about $50.00 US) is called Abacus Flight Simulator Design Studio v2. It is available here:
http://www.abacuspub.com/Just click on FlightSim Add-Ons in the upper right corner of the screen.
If you want to create a custom object to place into your FS 2004 Scenery this is the program that I recommend. Let's say that your home airport has a distinctive Terminal Building. You can create and texture this structure using FSDS v2. FSDS v2 also has extensive help files and instructions on how to create scenery items and place them into your FS 2004 world.
Are you into modeling and making your own aircraft? Abacus Flight Simulator Design Studio v2 will also do this.
Hopefully this will get you pointed in the right direction.