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. Turn them off during the registration, you can turn them back on after the registration.
Also, don't start FSX from another program, but start it from its default icon.
