Command not found is most likely due to /sbin not being on the default user path (it isn't on my CentOS 5.4 machine). You can just type /sbin/ifconfig. Or, you can change your path to include this.
As for networking not working... More details are needed. But, if you are using Gnome, I would go to system -> Administration -> Network as a first place to check things out.
Last edited by nuwen52; 03-03-2010 at 12:53 PM.