To be clear on this.
You would look for the /etc/apt/sources.list by going to your file browser and clicking on "file system" then "etc" then "apt" then right click on the file "sources.list" to view it.
A much easier way is to simply us one of these commands.
sudo gedit /etc/apt/sources.list
This is good for editing. You may need to change the "gedit" part of that so that it calls for your correct text editor.
This will let you look at the contents of the file in terminal and copy/paste it to, say, a post on a forum.
Your sources.list is what, in Debian based systems, controls the repos you have access to. Your error message was undoubtedly do to you not having the "public key" for medibuntu. This allows your box to check and make sure that the packages actually are coming from the correct place and are correctly signed by that place.
This is why it is best to only use official repos to get your packages. Medibuntu is considered one of the official repos by most folks. It has in it some packages that for one reason or another can't be in the regular Ubuntu repos. Could be that they do not meet Debian definitions of "free". Could, more likely, be packages that are not legal in some countries.
Ubuntu, or any other distro, can't be customizing repos for individual countries. It is up to you to know if the function of the packages is legal where you are or not.