Hi Jim. As you mentioned my name I add my two penn'orth.
The basic principle of installing aircraft to FSX is just the same as for other versions of MSFS. There is one important difference, FSX does not have an Aircraft directory. Aircraft are installed to the FSX\SimObjects\airplanes folder instead. Otherwise it's all the same as FS9.
You might like to have a peek at this tute.
http://www.simviation.com/fsxaircraftinstall.htmAs you had problems installing aircraft to FS9 I don't think you'll find this any easier. If you have problems post again with details of the aircraft & the exact page you downloaded it from. Then perhaps someone can walk you through it.