I was searching around on how to Enable Multisite on WordPress 3.0 RC1 and I couldn’t find anything, and some sites were incorrectly telling you how.
Here is the official instruction’s from WordPress on their Codex site.
http://codex.wordpress.org/User:Andrea/Create_A_Network
All you need to do is add the following code into your wp-config.php
define(‘WP_ALLOW_MULTISITE’, true);