Accessing the root account is disabled by default in Snow Leopard and the Directory Utility application is not available in the Utilities folder any more.
Here's how to enable root in Snow Leopard:
Warning: Enabling the root account can leave your Mac vulnerable to security threats. Only enable it if you are aware of the risks and know what you are doing.[robg adds: I've not had the root account enabled since the earliest days of OS X. If you do need to enable it, though, Terminal provides a simpler solution that appears to still work in 10.6: sudo passwd root. Enter your admin password, then a new password for root, and you're done.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090909081659323