Go to:
http://packages.medibuntu.org/
select your version in the list
then choose
libdvdcss2 from the list
then pick amd64 or i386 to download the .deb file.
Copy it to a flash drive, mount it in the ubuntu computer and navigate to the directory and
Code:
sudo dpkg -i libdvdcss2*
to install the package.
Then you should be able to watch DVDs with your media player.