Hi there,
first of all if you install an aircraft, simply select your Flight Simulator main folder, like "C:\Games\FS2004" and the installer program will put the needed files to the right place.
For panels, the situation may change. The most times a panel consists of several things, like the 2D graphics, gauges, panel sounds, general sounds, special effects... These are stored in subfolders, or in sub ZIP files. So after unpacking the panel package, copy/unzip the files according their names to the proper FS folder. Some examples:
Panel: copy these to the desired aircraft's own panel folder
Gauges: copy these to the main FS gauges folder
Sound: copy these to the main FS sound folder
Effects: copy these to the main FS effects folder
etc.
:s Usually every panel ZIP file has a readme inside that tells you how to install the panel properly, I hope this helps.