First off, welcome to the SimV forums!!  We hope to see you back here often!

Second, to do get a plane into your FS, download the file to a temporary folder anywhere on your harddrive.  Most downloads have installation instructions, but if they don't a good idea is to follow this procedure:

Open the .zip file

Create a folder with the name of the aircraft in the 'AIRCRAFT' folder inside the FS2004 folder

Copy the contents of the .zip file into the new aircraft folder

When you are finished, the new folder should have the following subfolders inside it:

model--with a *.mdl file and a model.cfg file
sound--with a sound.cfg file and maybe some sounds
texture--a set of textures (*.bmp)
panel--at least one *.bmp and a panel.cfg file

And it should have at least these files in it, although if there are more, it's okay:

aircraft.cfg
xxxxxxxxx.air (x's should be some name)

If all these files and folders aren't there, they may not have been included.  If not, check the place you downloaded it from and make sure there aren't other files that need to be downloaded with it.

Thanks guys,
That doesnt sound too hard.