The instructions for installing nmap, nmapFE, and other security auditing tools are pretty straightforward for OS X. However, Nessus, the #1 Open Source vulnerability scanner is not so easy. In keeping with my preference for Darwin Ports over Fink, I have created another 'How-To' on installing Nessus (and other tools) on OS X Panther using Darwin Ports.
The How-To is on my blog, jackburton.blogspot.com.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040710134036653