Am I correct in assuming that you have already unzipped the file, if it was a .zip file?
If so, is it a full aircraft package (ie. does it have a folder called 'model'), or just a repaint (a folder for 'texture').
If it is a full aircraft package, just copy the main directory into your FS\Aircraft folder, and select it the way you normally would in Flight Sim.
If it is a repaint, have a read of this thread, which is a guide for installing repaints:
http://nzff.org/forum/viewtopic.php?t=7