Just copy it to /etc/apt/sources.list,lets say this is the file,I'm going to copy and paste it,or like they suggest use the command line because they use sudo for some reason,you must be root;
just add that to the file anyway you can as root.Also you may want to put a ## in front of the universe and multiverse until you know what you are doing,on some distro's installing straight debian will break them because they are a mix of unstable and testing and sometimes experimental