There's absolutely no reason why the stand-alone Addon Manager would work, and the Zurich not.
Both installers installer exactly the same set of files, that are taken from the internet from the same location so, regardless of what you are installing, the files are just the same, just that Zurich installer has the scenery files as well, but they are otherwise identical.
Instead of keep installing and reinstalling, try to follow the suggestion that has been given: don't worry if the Couatl menu doesn't appear at the first start. Exit from FSX, start it again, and check the menu. Eventually, try rebooting Windows between tries.
Of course, assuming you already have correctly configured the antivirus (IF the antivirus is the culprit, of course), which means always installing with the antivirus real time protection turned OFF, and configuring it to exclude the FSX\fsdreamteam\couatl\couatl.exe folders.
