If you’re trying to get WordPress MU to work on a machine that isn’t on any domain, i.e. doesn’t have a dot in its name, this article by Will Norris leads you down a possible path.
To tie a bow on it, this is what I did to get things to work in a "WordPress MU subdirectory" configuration: In the wp-config.php file, add a line like this
define(‘COOKIE_DOMAIN’, ”);
In a subdomain configuration it probably won’t work.
fishercat
GeekTieGuy