There's no difference whatsoever between how a Serial Number is registered from Hawaii 2 and other sceneries, since the Addon Manager is only one.
If, after selecting "Register Serial" the Window doesn't appear, the error is due to another product you might have installed that "steals" the focus of the main FSX window, such programs are those related to online flying, since they force their own window to stay on top of everything else.
The registration dialog is probably there, just hidden behind the main FSX window.
Try to select CTRL+TAB or ALT+TAB to bring the registration dialog in front.