I don't know exactly what the problem is, but probably you don't know either so I'll tell you how I add aircraft and panels to FS
1.First unzip the file, not to the FS2004 directory as some manuals suggest, but to a directory on your desktop for example.
2.Now you will see two directories: Panel and Gauges. There may be more, like sounds, they are handled the same as the Gauges folder (check the readme.txt to make sure).
3.Drag the Gauges folder to the main directory of FS2004 so that the contents of the folder is added to the FS2004 Gauges directory (you can also drag the complete contents into the gauges directory of FS2004, just make sure you don't create a sub-directory).
4. The Panel directory must replace the panel-directory of the aircraft were you want to use the panel. Go to FS2004 and to Aircraft. Select the aircraft and you will see the panel directory which must be replaced. Drag the contents of your downloaded panel-folder to this directory. If you do it right, you should be asked "do you want to overwrite the panel.cfg?". Click yes!
1.Unzip the directory, again not to the FS2004 dir but to a self chosen directory on the desktop for example.
2.The directory should consist of four folders: Panel, Sound, Model and Textures (and maybe Effects). If there is only the texture-folder, you downloaded a repaint and need to download the full model, see the readme.txt where to get it. Make sure these four or five folders are in one directory which you name after the aircraft.
3.Now drag the main directory (consisting of these four or five directories) to your FS2004aircraft folder.
4.If there is an Effects folder, now drag the contents of this folder to the FS2004Effects directory.
5.Your aircraft is in FS2004 now, sometimes under the aircraft manufacturer name of IDFG, Project Opensky or something like that, so don't worry if you can't find anything under Boeing or Airbus.
This is the most basic way, it will work with most files but there may be exceptions. So always read the readme.txt files to see if you need to do anything different.
I hope this helps you!