STEP 1

Depending on your Ubuntu Kernel (Version) run the corrosponding command in your terminal:

• Ubuntu 9.04 “Jaunty Jackalope”:
  sudo wget http://www.medibuntu.org/sources.list.d/jaunty.list –output-document=/etc/apt/sources.list.d/medibuntu.list
• Ubuntu 8.10 “Intrepid Ibex”:
  sudo wget http://www.medibuntu.org/sources.list.d/intrepid.list –output-document=/etc/apt/sources.list.d/medibuntu.list
• Ubuntu 8.04 “Hardy Heron”:
  sudo wget http://www.medibuntu.org/sources.list.d/hardy.list –output-document=/etc/apt/sources.list.d/medibuntu.list
• Ubuntu 7.10 “Gutsy Gibbon”:
  sudo wget http://www.medibuntu.org/sources.list.d/gutsy.list –output-document=/etc/apt/sources.list.d/medibuntu.list

STEP 2

Once you’re run that in your terminal, run the following to get the GPG Key:
sudo apt-get update && sudo apt-get install medibuntu-keyring && sudo apt-get update

STEP 3

You've got your key - NICE! Now download the application:
sudo aptitude install skype

DONE =]