The install scripts already install ROS for you, these instructions are if you wish to manually install ROS.

For Ubuntu 14.04, you can just follow the normal ROS installation instructions.

For Linux Mint 17.x, the instructions are mostly the same, here are the steps (derived from http://insane-on-linux.blogspot.com/2014/10/installing-ros-indigo-on-mint-17.html):

sudo sh -c '. /etc/lsb-release && echo "deb http://mirror.umd.edu/packages.ros.org/ros/ubuntu trusty main" > /etc/apt/sources.list.d/ros-latest.list'
wget https://raw.githubusercontent.com/ros/rosdistro/master/ros.key -O - | sudo apt-key add -
sudo aptitude install ros-indigo-ros-base