Did you upgrade to 10.10? From the Medibuntu documentation on the
following page:
https://help.ubuntu.com/community/Medibuntu

--quote--
sudo wget --output-document=/etc/apt/sources.list.d/medibuntu.list
http://www.medibuntu.org/sources.list.d/$(lsb_release -cs).list &&
sudo apt-get --quiet update && sudo apt-get --yes --quiet
--allow-unauthenticated install medibuntu-keyring && sudo apt-get
--quiet update

Medibuntu's repository is deactivated by upgrading to a newer Ubuntu
release, so you should run this [the above] command again after the
release upgrade.

You may also wish to add the following packages. The first will cause
many apps from the Medibuntu repository to appear in Ubuntu Software
Center (Ubuntu 9.10+) or Add/Remove Applications (versions prior to
9.10). The second will allow users to generate crash reports against
Medibuntu packages and submit them to the Medibuntu bugtracker.

sudo apt-get --yes install app-install-data-medibuntu apport-hooks-medibuntu

--endQuote--

Even if you didn't upgrade, the command listed above should get the
repository installed for you. Probably best to copy and paste it from
the link I provided directly into a terminal.