I purchased all these 3 products when they came out - and I did install and use them without any problems, so it is not a question of a first timer having install issues.
That should in fact just prove to you that the installers works, because you *have* used the products in the past. What made a difference, is something in your system that changed between then and now. An online update of the antivirus might have caused this, for example.
I uninstalled everything and tried to install again, this time with antivirus off and added the whole FSX and P3D folders as exceptions for antivirus.
the Cuatl is not showing up in either, the add on manager is showing up.
Then you must have done something wrong, like not following everything in my previous reply TO THE LETTER because, if you had BOTH modules missing, it might have been some difficult to find issue with your Windows C++ runtimes libraries, or a problem with the Simconnect libraries.
If, instead, you are missing JUST Couatl and it's missing in BOTH FSX and P3D then it can only be the antivirus.
Redo all the following, exactly, don't skip ANY step. In FSX ( because P3D won't ask you ), PAY ATTENTION if you are being asked permission to run BOTH modules when you first start FSX. Obviously allow both of them.
If you REALLY followed all the procedure to the letter, without skipping ANY step, and you are not being asked permission to run BOTH modules, clearly indicate if you are being asked permission to run both, one of them ( which one, of course ) or none of them.
If you are not being asked permission to run *just* Couatl, then sorry, IT IS the antivirus.