Problem with locked-up plugin installation

I got installer happy and clicked to install the Signup Code Plugin before I had MultiSite enabled. Now I just get the following error when I try to login to my wp-admin page: "The Signup Code plugin is only compatible with WordPress Multisite." I've added the MultiSite "define('WP_ALLOW_MULTISITE', true);" code to my wp-config.php file and uploaded it to the server via ftp, and have reset my browser, but I still get the same error message instead of a wp-admin login pane. Any help is greatly appreciated!