How do I enable the "partner" repository?

GUI Way:

Click on the ubuntu button, then search for "Software Sources" (or "Software & Updates" on Ubuntu 16.04+) and go to the "Other Software" tab.

Command Line Way:

• Open the sources.list file: gksudo gedit /etc/apt/sources.list (or with the command line editor of your choice, sudo nano /etc/apt/sources.list would use nano on the command line without a GUI)

• Add the partner repositories by removing the # in front of the following lines (maverick is the version of your Ubuntu installation, it may differ, so use the codename of the release you are using instead of 'maverick'. If you're not sure run lsb_release -c to find out.)

  # deb http://archive.canonical.com/ubuntu maverick partner
  # deb-src http://archive.canonical.com/ubuntu maverick partner
  

• Save and Close.

• Resynchronize the package index files from their sources: sudo apt-get update

• Official documentation for reference

The simplest way to enable "partner" repository:

sudo sed -i.bak "/^# deb .*partner/ s/^# //" /etc/apt/sources.list
sudo apt-get update

CLI method

This method uses

• lsb_release -sc to get the Ubuntu (codename) version.
• add-apt-repository to alter the appropriate config files.

It has the advantages of working in all versions of Ubuntu.

sudo add-apt-repository "deb http://archive.canonical.com/ubuntu $(lsb_release -sc) partner"

This is from a skype tutorial