The easy way to find downloads on Simviation is to use the seach option. Type in : fs2004 md83 and other options can include md80, md82, md87, md88 and md90. If you are unfamiliar with downloading and installing aircraft check out Grump's Lair on the Simviation homepage.
Whatever you download for FS2004 always have a seperate directory/file outside of the game to put them in. For me I have a main download file with many sub-directories for different aircraft, sounds, panels, etc.. Almost all downloads are zipped files that must be placed in specific locations. Some downloads have automatic installers but don't count on it. The AVSIM website also has a large collection of SGA MD-8x aircraft.
Some painters can really test you. All they will do is give you an aircraft texture & minimal supporting documentation. Developers can release aircraft in a multiple of generations that may use different models or .air files. You are sometimes left to try and figure it out the specifics yourself. These mistery textures have a high

factor and are best usually avoided.
http://www.armonts.net/fsairdsgn.htm - A collection of links to the best free/payware aircraft out there!

Best of luck
Highfive