It turns out the upgrade of rc3 has a small typo in the upgrade. New installs are not affected. Upgrades from any earlier version are.
To fix this run the following 2 queries in PHPMyAdmin:
ALTER TABLE `wp_adrotate_groups` ADD `page` LONGTEXT NOT NULL ,
ADD `page_loc` TINYINT( 1 ) NOT NULL DEFAULT ’0′
Make sure the columns are added in at the end of the table. Without these 2 columns creating new groups does not work. The typo has been corrected and RC4 will have the fix when it’s released.