Somestimes I just don't understand...

Sometimes I just don't understand why a plane with all the necessary files will not show up in the list of selectable aircraft. I downloaded, unzipped and installed a Grumman AgCat from another site. The plane has every requisite file needed. It unfortunately has that little brown parcel box I got after I got the latest Adobe Reader thingy, but I have put in other planes since I got that thing, too. This plane is supposed to be FS2004 compatible. It does not show up, but another AgCat (from the same site) for use in FSX shows up and functions perfectly well. I want the FS2004 one because it's painted just like one from my youth of 40 years ago. Why is it not showing up? It has all four files, the .cfg file and the .AIR brown box installer thingy. It should be there, but it ain't. I have many others that have exactly the same set-up and they are all there, so what's the deal? 
