Check you are not running another add-on that is launching FSX itself. Launch FSX from its icon
Be sure you don't have another add-on that is "stealing" the focus, so it forces the registration dialog behind the FSX one (the dialog is likely there, you just don't see it because of this other program). Programs that are known to cause this problem are online flying clients with chat windows and similar, or programs that launch FSX themselves.
Turn them off during the registration, you can turn them back on after the registration, and launch FSX from its own icon.
