If you're trying to do what I think you're trying to do, you can do it in System Preferences/Network if you select manual dns configuration. (It's the search domains.) Unfortunately it seems to be broken and only the first one entered is consulted.
If you need to have specific entries (as in /etc/hosts), you have to use the NetInfoManager. It's the machines subdir.
Choose "New Subdirectory"
Change the name to the name of the machine.
Choose "New Property"
Change the name of the property to ip_address
Choose "New Value"
Change the value to the machines ip number.
I've heard there was once a Network Manager that did these thing much easier, but it doesn't seem to be in beta.