How to install mkusb in Ubuntu 14.04.1 LTS and newer versions
This method applies to newer versions too: 14.04.2, 14.04.3, 15.04, 15.10. (Notice that 14.10 has passed end of life.)
Getting the repository Universe
This command line in a terminal window or text screen
sudo add-apt-repository universe
will 'enable the universe distribution component for all sources', in other words 'install' or make available the repository universe.
The fastest way to start making USB boot drives is to install the mkusb PPA, install and update the mkusb package like all the other program packages. See this link
sudo add-apt-repository ppa:mkusb/ppa # and press Enter to accept it sudo apt-get update sudo apt-get install mkusb sudo apt-get install mkusb-nox sudo apt-get install usb-pack-efi # if you want to make a persistent live drive
The whole dialogue in a terminal window is shown in the attached pdf file. It will install pv (alongside mkusb), if you have previously installed Synaptic or some other program in order to install the repository (alias source) Universe.