You need to use a text editor to be able to change your sources file.
Go into a console and type, first :
su --> then your root password.
Once you're root (you know this because there is the # sign at the prompt, type :
vim /etc/apt/sources.list --> that's what you did, but perhaps you were not root at the time.
now press "i" on your keyboard --> that puts you in the "insert" mode. You will see it at the bottom.
then scroll down to
etch main contrib non-free
are to be inserted here. type normally, don't insert bold type
Once this is okay : press "esc" to quit the insert mode.
Once done, type ":wq!" this will write (w) quit (q) immediately (!) once you have pressed enter. Don't type the "", of course ;-)
Congrats ! you've successfully modified your file, using the best texte editor there is (if a little complicated at first)
Run apt-get update
Run apt-get msttcore
Watch it install.