Hi everybody!
Well, I was configuring a web server, that is why is not using DHCP.
Also, I believe is not good idea to try to hack directly into the flat configuration files (/etc/*) in a MacOS X system, as it seems that even MacOs is a UNIX like OS, the configuration of it works in a different way!

So, most of the tricks from other OS's do not work here because of the infamous NetInfo!!
Any way, I finally found that the command I was looking for is /usr/sbin/networksetup. In fact, I realize that the administration command that every MacOS administrator should know are:
/usr/sbin/networksetup
/usr/sbin/systemsetup
/usr/sbin/softwareupdate
/usr/sbin/installer
With this commands you can achieve some of the most important administartion activities on a MacOS X server system.
BUT, I do not know why it seems that these commands only work on the 'server'edition!!!
Regards!