First, login to the VPN.
Now add the PPA as follows (replacing [NAME-OF-PPA] with the desired PPA):
echo "deb http://archive.admin.canonical.com/ubuntu [NAME-OF-PPA] main" | sudo tee -a /etc/apt/sources.list # Add precise-cat-proposed to sources list sudo apt-get update # Update apt for new sources
And install whatever you want.
To get a list of existing PPAs, you can go to http://archive.admin.canonical.com/ubuntu/.