This video screencast is a quick tutorial for how to set up WordPress 3.0 in multisite mode. The WordPress codex has written instructions if you want to use those as well.



[youtube]http://www.youtube.com/watch?v=wZ7ZizdF_i4[/youtube]

Update the wp-config.php file

You will need to add this line of code to your wp-config.php to make the multisite function accessible:

define('WP_ALLOW_MULTISITE', true);

Links

WordPress codex documentation for using multisite:

http://codex.wordpress.org/Create_A_Network

How to map top level domains in multisite:

http://verysimple.com/2010/07/09/install-wordpress-3-with-multiple-tlds/

http://ottopress.com/2010/wordpress-3-0-multisite-domain-mapping-tutorial/