Sure I can tell you how to do it, though I use FS2004 the process for FSX is very similar:
1. You have to download the plane, the best sites are flightsim.com, simviation.com, avsim.com (closed now

). Make sure the plane you are downloading is not a repaint but the actual model
(e.g. the UA 744 on this page is the actual model with textures included)
Flight Simulator Downloads - Microsoft Flight Simulator X, FSX, FS2004 FS2004, FS2002, FS2000, FS98, Combast Flight Simulator 3, CFS3, CFS2 & CFS Add-ons!
2. Once downloaded you take the folder containing all the files like aircraft.cfg, a texture folder, a model folder and copy and paste to:
C:\program files\microsoft games\flight simulator x\sim objects\airplanes
NOTE: sometimes you have to extract the file and your FSX main directory may be somewhere else other than the default (C:\program files\microsoft games\flight simulator x)
And I'm not sure if FS2004 aircraft will work in FSX but I do know that there are a lot more aircraft availible for FS2004 than FSX.
Good luck and if you have anymore questions just ask!