I'm out of touch with the latest methods. API macros were originally used with the Airport for Windows scenery design application. http://www.airportforwindows.com/
They can also be used with FS Scenery Creator. http://www.fssc.avsim.net/
Both work in much the same way. Import the API macro & place it into an existing project. You can use the same macro more than once or any number of different macros in a specific project.

An alternative method would be to convert a 3rd party aircraft & export it in BGL format with MDLtoBGL. http://www.avsim.com/hangar/utils/mdltobgl/id2.htm
Older FS98 designs work best with this utility. It will also export in API format.

I'll give you a quick walkthrough in FSSC from memory. First define the airport from Edit/Scenery Properties filling in the details. Airport name, co-ords etc. Then right-click anywhere on the screen selecting New 3D Object/Macro from the dropdown menu. Navigate to the folder where your API macro is located & select it. Position it where you wish & repeat the process to add more if required. Save your work at this point. Finally export in BGL format from File/Export Scenery Wizard.

I think MDLtoBGL might suit you better.

Hiya

Because SCASM scenery (when API macros are placed using programs like FSSC and Airport) will not be used after FS2004 everyone is now moving over to XML scenery tools like RWY12, AFCAD etc. This means that you will have to use default library objects, objects created in GMax or FSDS2 by other people and placed in libraries for you to download and access or objects created by yourself.

However, there is an alternative. Tom Hiscox has seen that in the meantime while FS2004 is still in use and SCASM objects can still be used, it would be useful for designers to be able to make use of BOTH technologies - XML and SCASM.

Therefore he has created a UNIQUE product (as far as I'm aware). The latest version of SceneGenX does more than any other program. It lets you create aprons, taxiways, parking etc (like AFCAD), lets you place XML objects (like RWY12) AND uses Airport technology (which IMO is better than FSSC anyway) to place API macros.

I've been using it to update my Kai Tak scenery as at the moment I haven't had the time to learn GMax (and probably never will do so my scenery design days are now numbered). As a 'hybrid' it is therefore the 'ideal' system right now but I have to admit that it does have a few quirks that you have to get used to. Anyway, it has worked for me and I recommend it as well worth a look.

Anyone who is working exclusively now in FSSC (or indeed Airport) is really wasting their time, but if they can't use GMax to create their own objects IMO they are also restricting themselves considerably by using AFCAD and RWY12.