$ sudo apt-get update
$ sudo apt-get install ettercap-graphical 

ettercap-graphical is installed in your system.

Make ensure the ettercap-graphical package were installed using the commands given below,

$ sudo dpkg-query -l | grep ettercap-graphical *

You will get with ettercap-graphical package name, version, architecture and description in a table

ettercap-graphical package basic information: