I installed the latest add on manager and everything works as it should until I push the register button on any Flight Beam airport. FSX pauses, but no pop up window for activation shows up.
If you don't see the registration dialog, the problem is never the Addon Manager but, it's another add-on you have that has "stolen" the window focus to force itself on the foreground, so the dialog is probably there, but you can't see it because it has been sent behind the FSX window by such other add-on.
Programs known to cause this problems are usually online flying clients with chat windows that force themselves on the front.
Close all of them to do the registration. And/or, use CTRL+TAB to bring the registration window back on the foreground.