FSX add on Aircraft downloaded

I have recently installed FSX and it is working quite well . But I do have a problem with it that when I download add on Aircraft I cannot get them to work in the actual FSX game . I put the files into a sub folder in the aircraft folder which is in Sim objects, along with all the aircraft put there by Microsoft. But I cannot find anywhere in the game where they can be activated by FSX you can inport scenery but nothing for extra Aircraft. You must be able to do it somehow, is there anyone out there who knows where the magic switch is? I have tried E-Mailing the authors of the downloads and variuos microsoft E-Mail addresses but I have had no replies.
Best regards Madmax (not really only a little bit)
Best regards Madmax (not really only a little bit)