Check you are not running another add-on that is launching the sim itself. Launch the sim 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 simulator 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 the sim themselves.
Turn them off during the registration, you can turn them back on after the registration, and launch the sim from its own icon.