Hi Bill,
That's correct; to echo Bill Womack and others, the Orbx addon airports are not designed for Default FSX, but our base terrain addon regions, such as PNW. You are welcome to install them over Default FSX but they won't blend into their surrounds and you'll also miss out on a lot of the tech like 3D lighting and correctly aligned roads, rivers, shores and custom autogen.
So it's best to purchase and install PNW and then grab 3W5 while the July promotion is on to give you WA56 for free. I can also recommend Bill Womack's 7S3 Twin Oaks, and we have a freeware airport 2S1 Vashon Muni available from
http://fullterrain.com/freeware.html
Also, the team over at OZx are about to release a whole bunch of beautiful photoreal freeware airstrips for PNW.
There's plenty to do now, and a whole lot more to come very soon.
Hope this clarifies.