yup, it should be available from add-widget option

well i can't think of any other solutions. but maybe, just maybe, you've somehow have added it to your desktop (it wont be listed in add-widget once you've put it to your desktop) but in different desktop?
i mean if you've installed some tools that can save/restore your desktop setting (i forget the name) and it has been shown in one of your saved desktop, so it probably won't shown again