Ola Edcarlos, e so dar o comando no terminal;
Adding the Repository
The following bash command adds Medibuntu's repository to Ubuntu. It also adds Medibuntu's GPG key to your keyring, which is needed to authenticate the Medibuntu packages.
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This command should be run in the Terminal (Applications → Accessories → Terminal):
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 update && sudo apt-get --yes --quiet --allow-unauthenticated install medibuntu-keyring && sudo apt-get update
Se eu nao estiver enganado o Skype nao esta mais nos pacotes do Mediubuntu
http://packages.medibuntu.org/karmic/index.html sendo necessario o download direto do site;
http://www.skype.com/intl/pt/download/skype/linux/