If you are missing the Addon Manager or the GSX menu (or both) from the "Addons" menu of the sim, the only possible reasons are either the antivirus blocking the update, or a problem with your VC++ libraries, which might be missing/corrupted, even by something not related to flight sim.
Disabling the antivirus while installing its NOT enough. In addition to that, when the antivirus is suspected to be the cause of the problem (it's the more common one), you MUST also configure it to exclude the Addon Manager folder before starting the sim.
About missing/corrupted VC++ libraries. They are the 2005 SP1, the 2013 and the 2015 version. Our software doesn't require all of them but, indirectly, Simconnect itself requires it so, if one of them it's missing, you'll have lots of issues, and not just with our software.
The linked thread named "Addon Manager not showing up in P3Dv4 **SOLVED**, was quite exhaustive:
http://www.fsdreamteam.com/forum/index.php/topic,17484.msg122656.html#msg122656
