Probably caused by privacy settings. I changed the error message guiding you to check the privacy settings (allowing the extension to interact with secure pages). Change was made in version 1.6.0 (update will be available in next few days)
What version of Opera are you using? Do you have some error messages in console? You can open the console using Menu -> Page -> Developer Tools -> Error console (ctrl+shift+o), then clear all error messages and try to add the page - if any error messages appear immediately after adding the page, please post it here.
After clearing, then trying to add a page that can't be added, the Console says:
[25/09/2011 00:03:34] JavaScript
Event thread: click
Error while getting tab info
[25/09/2011 00:03:44] Widgets
Widget loading warning: element not fully defined.
[25/09/2011 00:03:44] Widgets
Widget loading warning: License href='(null)' is neither a valid IRI, nor a path within a widget - ignoring this attribute.
[25/09/2011 00:03:44] Widgets
Widget loading warning: License href='(null)' is neither a valid IRI, nor a path within a widget - ignoring this attribute.
P.S> Third attempt at posting this. Your Captchas are too difficult. I am a human. I am not psychic.