Enabling WordPress Multisite - Updating your wp-config.php file

Hi All, thanks for the help earlier - invaluable!
Another one for you all ... I've got further through the installation process and now stuck on enabling wordpress multisite - updating your wp-config.php file.
I can't get WordPress to recognise the line = define('WP_ALLOW_MULTISITE', true);
I've tried all the fixes listed in the instructions but still no joy!
All help gratefully appreciated xx