Ok this is driving me nuts now....
Having the same issues, uninstalled it, reinstalled it with no AV..... same problem.
uninstalled all FSDT and FlightBeam scenery, and reinstalled offline and without AV online. Got it to work! FSX asked to trust Couatl and Add on Manager.
Now today same thing happening! uninstalled KSFO and the add on manager a Couatl, reinstalled offline without AV and restarted FSX and said yes to trust Couatl and Add on Manager, and still same problem! Now Couatl appearing in the add on manager list.