I havent got my email open at the moment, so I'll give the instructions here, and then you can copy and paste into any program like notepad, and then print off to read at your leisure.
Ok, if I miss anything out, just correct me guys
Firstly, open the .zip file, with Winzip preferabbly, although there are other programs out there. Winzip is free to use as a trial, but I've never registered because you just have to click 'use evaluation version' when you open it. Now, making sure that no files are selected, click 'extract' (because no files are selected it will extract all the files in the .zip file. If you selected a file, then it would only extract that one). Now, it will come up with an extract window, where there will be a path, i.e. C:\Program Files, or something along those lines. Enter, or use the browsing tool to find, a directory you want to temporarily put the files. Then click extract. Winzip will work for a few seconds, depending on the size of the .zip file. Then, go to where you extracted the file (click on my computer, C:\ etc or whatever drive you used) and find the extracted files. Now, if there is a folder there, probably named according to the aircraft downloaded, open it. Inside, there should be a folder 'Sound' 'Panel' 'Model' and 'Texture', but it might also have something like 'texture.1' 'texture.2'. Dont worry about that, thats just different liveries for your aircraft. There should also be an aircraft.cfg file and an aircraft.air file. Now, you can copy the entire containing folder to you ....FS9\Aircraft folder. So in your FS9\Aircraft folder, you should have a folder containing the model, sound, panel etc folders. This should then be done. Note, there may be another folder saying 'manual' but that is for you to read is you wish, and there may be 'gauges' and 'effects' folders, and the contents of these should be copied to the 'guages' and 'effects' folders found within your main fs9 directory.
Hope that helps, although not hugely clear. In the .zip file, there is usually a readme.txt file. Open that, and it will give you installation instructions for that aircraft. Alot of the time you can extract the whole .zip file to your fs9\aircraft folder and everything will be done. Then open up fs2004 and everything should be there