Wehn I open the Addon Manager and klick "Register Serial" no window appears, wehere I can insert my serial. FSX is not running in full screen mode.
Check you don't have another program that is stealing the focus from FSX.
Maybe the dialog it's there, but it has been sent beneath the FSX window by another product that has taken the window focus. If it's there, it should be possible to put it back in front, by using the standard Windows CTRL+TAB key combination.
@Bruce
If the product was still registered, the "Register Serial" button would be greyed out and not even clickable.