Hey everyone!
I have a few problems with Seattle airport, firstly fsx atc seems not to use 34L/16R as much as it should (default scenery). Also, does anyone know any scenery for Seattle I can't seem to find any, and if I do, will it mess up WOAI and will atc use all the runways?
Any help would be greatly appreciated. Sorry if this is in the wrong place.  :)

FSX uses a 'scoring' system to determine runway assignment, but in the end the primary factors are:

Wind direction (over 16 knts I believe) determines runway(s) in use.

If parallel are in use........

For arrivals-direction from which aircraft is arriving, i.e. (KSEA) from the east, 16L/34R; from the west , 16R/34L.

For departures-runway nearest the parking spot that meets minimums for the aircraft.

For KSEA downloads here at SimV: http://simviation.com/1/search?submit=1&keywords=ksea&categoryId=54. The 3rd one, KSEA.zip, adds the 3rd runway and taxiways to default scenery. For others, look here: http://www.freewarescenery.com/fsx/us.html

Airport downloads should work fine WOAI. They generally will not change runway usage....

(Added-[timestamp=1357833378])...though  there are 'tricks' that designers can use, most often with parellel runways. KSEA.zip, that I mentioned above, has takeoffs limited to 16L/34R with landings on the other 2 parellel runways. This does help congestion during busy times.