If your running Ubuntu you should have some type of file editor installed already. I would assume that it has that loaded.
When you open your terminal you will be a local prompt, if your paths arent set you will not be able to access the file directly.
you will need to
sudo vi interfaces
It will bring up the file in the VI text editor. Hit the insert key and then arrow down to the last line and enterFOG Server IP (This is just a remark to remember what your doing)
auto (the name of the NIC you are using. You can use IFCONFIG to view the NICS in the machine) in my case its
iface p4p1 inet static
address (your server address)
netmask(Your server netmask)
gateway(Your server gateway)
It should be some form of this. Hope this helps.