if you don't know how to enable root, you probably shouldn't be ... LOL
nore should you be using nmap
😉 --- to many hacker movies my friend ... LOL
i'm just teasing.
I have this working in 10.4 but if i recall I had to install X11 and to the whole nmap/zenmap install right in the root user account.
SOOO
http://www.macosxhints.com/article.php?story=20071025100950309
use that link to enable root, then log out of your account and login to the root user
username - root
password -- whatever you decide
then try installing it... best
www.nmap.com has a version that should install no problem -- but again .. that's me on 10.4 not 10.5