For those official aircraft, you'd maybe find it easier to use the hangar system: check the 'official FlightGear aircraft hangar' is listed in the top section of the 'Add-ons' page. (If it's not listed, click the button to add it) Then you should be able to install and use them directly: FlightGear will download them for you and ensure they're in the correct place. You can do all that from the 'Aircraft' page.
If you do prefer to download aircraft yourself, you need to extract the zip contents, i.e you can't just put '747.zip' into a folder like Documents/MyAwesomeAircraft: you need to extract the zip to get a folder called '747' and put
that into your Documents/MyCoolAircraft folder. And then you add 'Documents/MyBrilliantAircraft' to the list of 'additional aircraft folders' in the 'Add-ons' page. (What you call the aircraft folder is up to you, I'm just giving some examples...)