Exclusively 2 Screenshots of the new WordPress 3.0 Multisite Settings. You can find them under Options->Network:
After you set the settings, you receive in a second step the content for wp-config.php and .htaccess. Additionally you have to create the subfolder blogs.dir in wp-content , where you put the media-files for each blog.
Since WordPress 3.0 is still in development, there might be many changes until the final release.