Hi!
Oh. The thing you changed effectively locks you out of doing anything in the admin panel. It's only there to dumb down a user account for foreign users, that should only log in to be able to post comments to otherwise locked entries. The dangers of this setting lead us to remove that setting in the personal configuration in Serendipity 1.2 versions for the upcoming release
Best would be to reinstall serendipity, if you didn't already add content to it: Delete the serendipity_* tables, delete the file serendipity_config_local.inc.php and you can reinstall.
The other way would be to edit the serendipity_config DB table to reset the option "no_create" to "0" instead of "1" for your authorid.
Best regards,
Garvin