[OSGeoLive] v13 VirtualBox can't open terminal or firefox

[Felicity Brand]

Hello,

I have a working VM with OSGeoLive v13 on it. Yay! Unfortunately I
can't launch a terminal window or Firefox. The cursor shows a spinning
wheel for a while then nothing. I'm new to Linux so hoping someone can
help me.

I got to the CLI and tried `sudo apt-get build-dep gnome-terminal`
and the response I got was "E: You must put some 'source' URIs in your
sources.list"

Not sure where to from here.

Thanks in advance
Felicity