you might want to add this using phpMyAdmin:
Code:
insert into configuration (configuration_title, configuration_key, configuration_value, configuration_description, configuration_group_id, sort_order, last_modified, date_added, use_function, set_function) values ('Define Page-Not-Found Status', 'DEFINE_PAGE_NOT_FOUND_STATUS', '1', 'Enable the Defined Page-Not-Found Text from define-pages?<br />0= Define Text OFF<br />1= Define Text ON', '25', '67', now(), now(), NULL, 'zen_cfg_select_option(array(\'0\', \'1\'),');