Is there a right and sure way to add FS2004 aircraft to FSX

I have attempted dozens of times to add FS2004 Acft. to my FSX Sim Objects and they do go in there but they never seem to be useable? Reading install directions are very weak in most cases and appear to be fine for the professionals. Any suggestions? I have no problem installing FSX aircraft most of the time.
Any advice would be greatly appreciated!
Rudy