Ok, it seems I found the problem, and it wasn't with the anti-virus. You may want to remember this as another possibility of the problem.
Just on a side note, the downloader and installer worked fine in Safe Mode. Just make sure Internet access is enabled with Safe Mode. It ran and placed all the files where they belong. A sure fire way to get around any anti-virus issues.
After running the installer for the Addon Manager once for Prepar3D and once for FSX, I had failure of the Couatl engine in Prepar3D. I wondered if the same was true for FSX, so I fired it up. When FSX booted I got three messages asking if I trusted the Virtuali Addon Manager, Couatl, and something else I missed. Naturally I selected "yes" to all the prompts of trusted software. In FSX I had the Addon Manager and Couatl working, no problems. I didn't recall seeing any of the same prompts when starting up Prepar3D, (if I trusted your software). This got me thinking about permissions. In FSX, I was being asked if I trusted your software, in Prepar3D I was not. In FSX the Couatl engine worked, in Prepar3D it did not.
I have full administrative rights on my system all the time, and both my FSX and Prepar3D installations reside on a separate drive away from the default C:/. I opened the the Fsdreamteam folder and navigated to "Prepar3D/Fsdreamteam/couatl/couatl.exe" and right clicked it and selected the "Properties" tab. I then selected the "Compatibility" tab and checked the "run this program as an administrator" checkbox, selected "apply" then "OK" and closed the window. This time when I ran Prepar3D, the menu had the Addon Manager and the Couatl Products tabs present and all the FSDT airports functioned properly.
In my FSX installation, I have not had to do this, perhaps because during initial boot of FSX, it is permitting their use because as an "Administrative" user allowing the files to be determined as "Trusted". In Prepar3D however these "pop up" Windows do not appear, it goes straight to loading the scenery and terrain and then into the simulation environment. This never allows the user do determine the "Trusted" factor of any of these .exe files. By habit, I always check the box of any .exe file to "run this program as an administrator", regardless of what drive the file resides on. This has gone a long way to prevent unexpected problems of programs. Usually when a file is overwritten in retains the set properties, which is why in the past the Couatl.exe even though updated always functioned properly. For some reason this time, perhaps because of the install of multiple new airports the setting was removed. Even having full Administrative privileges over the system was not sufficient enough for the Couatle.exe to run. As soon as I checked the box, it instantly worked.
So in closing I would also recommend that when the issue crops up that anti-virus be turned off during download and install as a safe guard, but also that the "run this program as an administrator" check box is checked as well in the "Compatibility" options, particularly in Prepar3D as this appears at the moment to be a must for functionality.
Fact you can't even fully deactivate Norton in normal mode, only adds up to my suggestion to switch to a more reliable product, like the free MS Security Essentials, which doesn't mistakenly identify our software as a threat to begin with, which means it doesn't require any special configuration, it just works.
I know you hate Norton with the red hot intensity of a 1000 suns, but there is good reason for this. A little history reveals why. Several years ago a virus was created that not only harmed ones computer, but was designed to mess with Norton. When it was downloaded in a file, the first thing it did was disable Norton silently in the background. It then tinkered with the files to give the appearance to the user that Norton was up and running, when in fact it was not, and even go so far as to show that "updates have been downloaded and installed" and that the user was now up to date. This too was false. Meanwhile, the virus delivered it's payload and began it's nasty little work on your system. I only downloaded what I determined as "safe" material yet this little sucker found it's way onto my system. I only discovered it when my Internet was down and I still received the message that updates had been downloaded and installed and that I was now up to date. Really? With no Internet connection! I shut my system down. When my Internet was up and running, I opened the Norton control panel manually, and manually selected "Run Live Update", which was followed by a huge amount of content, then ran the scan, and low and behold it found the virus and sent if packing. The conclusion reached by Norton was that if you or I can disable the module, so too can a hacker or cleverly scripted virus. The solution was to make it impossible for it to be shut down, other than by un-installing it. This is still a real vurnability any anti-virus has that can be totally and completely disabled. Noton 360 is extremely robust and I must admit, your content is the first in 10 years of use that I have had conflicts with. It is true, that this is not accepatble considering you have sent them the offending files and have recieved no "love". I will contact their customer support and will also give them a nudge as they proclaim to detest false positives, well now I'm going to call them out on it, and hopefully end this problem for good, at least with Norton anyway.