You will first need to download an aircraft addon file from SimV's file library, located here:
http://www.simviation.com/simviation/Once you find the file that you want, download it, and unzip it with a software program like Winzip or 7-zip. A complete aircraft package should include a the following folders in a main folder for the aircraft which lists the name of the aircraft:
Main folder:
subfolders-:
model
panel
texture (if multiple textures, more folders such as texture.1, texture.2, etc.)
sound
In short, FS2004 aircraft need to have a basic airframe (model) folder located in the addon aircraft's folder for a plane to show up. The textures, and any addon textures, must also be compatible with that author's work.
Simply loading in a folder full of textures and a panel will not result in a flyable aircraft.