I am really having a hard time when I download new aircraft. I get thru every step fine but when I click on FSX and choose the new aircraft, it's not there. What am I doing wrong? i am really getting frustrated because I am unable to fly various aircraft Simviation has to offer. 
I am following the directions and putting the aircraft in the correct  (sim objects, aircraft) files but to no avail.

Can someone tell me if there are certin aircraft I can download to FSX and what I should look for?

Thanks,
Eyeman22