I know your problem, been there done that. I will assume you know as much as I knew when I started.
You will need to write just 3 lines in the terminal; here I will start the lines you need with the prompt $ so you just write what follows $, for your benefit, any comment I will start with # they are for clarification and you don't type.
# the first thing you need is to edit the sources.list file; the commad is
$gksudo gedit /etc/apt/sources.list
# that command will run gedit and open the file sources .list that resides in /etc/apt/
# gedit will open in different window so you go to the editor window, add the line
# once you add the line, you save the file, close the editor, and return to the terminal
# when you are back at the terminal you have to update your source list and then you have # to install enna. You will use the following two command lines
$sudo apt-get update
$sudo apt-get install enna
# That should do it for you.
Good luck and happy Ubuntuing