If you don't have the whole "Addons" menu, your problem is way more deep that "just" the Addon Manager or GSX. Unless you have just reinstalled FSX from scratch, and haven't installed anything else, chances are that menu should contains several entries from other developers modules, like the popular FSUIPC which is used by many products, but there are many out there, from PMDG, Wilco, Aerosoft, Level-D, etc.
So, it would be good to know if you have just installed FSX and the *only* modules you have are the FSDT ones.
Turning off the antivirus while installing it's not enough: if the antivirus is the problem, it will try to block modules when you start FSX, the correct way is to BOTH turn it off when *downloading* AND *installing* AND configure it to exclude the fsx\bglmanx.dll and fsx\fsdreamteam\Couatl\couatl.exe files from scanning BEFORE trying to launch FSX the first time after the installation.
And, you should take note if you ARE being asked permission to run the Addon Manager and the Couatl modules the first time you start FSX after the installation. If you haven't, then something is wrong and, either the antivirus is still blocking them OR something is missing in your Windows install, like the VC++ 2005 runtimes.
Note that, it's not very clear what you mean with "I've recently installed", can you try downlading the Stand-Alone Addon Manager NOW and install that one ? That one gets updated very often.
