That is strange. I had been playing with the installers on several occasions over a period of about a week and my last attempt at a fresh install resulted in a good install without having to edit my interface file. Any way, start by opening a terminal window on the core/hybrid (by pressing ctl shift f2 this will switch you to a terminal window), type cd /etc/network at the command line, and press enter. This will change your current directory to /etc/network. Within this directory you will find a file called interfaces. Verify that the file needs to be edited. Type cat interfaces. This will display the contents of the interfaces file. On my machine the file displayed the following:
iface lo inet loopback
iface eth0 inet static
iface eth0:1 inet static
Originally I was using a single network card in my test installation. I have since added a second card. This could have been part of my problem but I am not sure. If you have no netmask or gateway entry in your interfaces file then you will need to add them. I used vi to edit the file but it is very difficult to use if you have not used it before. You should have pico on your system. Type sudo pico interfaces. Enter your password when prompted. This will display a full screen editor that is fairly easy to use. As you can see there was nothing entered for my netmask or gateway entries. The ip address, in my case, was a problem since it is listed as static and it is from the range of dhcp addresses on my network. Just add the necessary information, change the ip address if needed, save the file, and then exit the editor. Reboot your machine.