For Linux/Mac skip step 4 and 5. Those are only for Windows.
1 - First, make sure you have Flightgear stable release version 2016.1.1.
2 - Start up flightgear from the default shortcut, do not use FGRun or other launchers. (fgfs.exe --launcher)
3 - Go into settings and then "Configure add-on aircraft and scenery"
4 - At the top there is a button "Change...", click on it.
5 - Choose the folder "c:\Users\[win username]\AppData\Roaming\flightgear.org" and click "Select folder". (as the default folder wont allow downloads to work[a win only bug I believe) [notice this will also move your Terrasync folder to there]
6 - Under Aircraft hangar locations, click the "+".
7 - Enter: "http://fgfs.goneabitbursar.com/official/catalog.xml" and click "Next". (and wait for it to finish)
8 - Click "OK". Then click "Quit"
9 - Start up flightgear again (same manner as point 2)
10 - Uncheck "Only show installed aircraft". And enjoy seeing 400+ aircraft in your launcher, ready to download and fly.
11 - We are not finished yet, hold on.
12 - Select an installed aircraft and click "Run"
13 - In the menu "File", select "Aircraft Center (experimental)"
14 - Click "OK" to the question.
15 - Scroll down and select an aircraft, and click install.
16 - Wait for install to finish.
17 - You now need to refresh the page, just click on "installed" tab at top.
18 - You should see your newly installed aircraft now, and a button underneath the ratings. Click that button and enjoy.
Ofcouse you only need point 1 to 10 if you want to install/run from the new launcher, else do to point 18.
Once you have done this, you don't have to use the new launcher anymore. It should just work.