Configure your Ubuntu repositories
Configure your Ubuntu repositories to allow "restricted," "universe," and "multiverse." You can follow the Ubuntu guide for instructions on doing this.
Setup your sources.list
Setup your sources.list file to accept Debian packages from the ROS server.
Ubuntu 9.04 (Jaunty)
sudo sh -c 'echo "deb http://code.ros.org/packages/ros/ubuntu jaunty main" > /etc/apt/sources.list.d/ros-latest.list'
Ubuntu 9.10 (Karmic)
sudo sh -c 'echo "deb http://code.ros.org/packages/ros/ubuntu karmic main" > /etc/apt/sources.list.d/ros-latest.list'
Ubuntu 10.04 (Lucid)
sudo sh -c 'echo "deb http://code.ros.org/packages/ros/ubuntu lucid main" > /etc/apt/sources.list.d/ros-latest.list'
Ubuntu 10.10 (Maverick)
sudo sh -c 'echo "deb http://code.ros.org/packages/ros/ubuntu maverick main" > /etc/apt/sources.list.d/ros-latest.list'
Set up your keys
wget http://code.ros.org/packages/ros.key -O - | sudo apt-key add -