$ sudo apt-get update
$ sudo apt-get install libmonogame-cil 

libmonogame-cil is installed in your system.

Make ensure the libmonogame-cil package were installed using the commands given below,

$ sudo dpkg-query -l | grep libmonogame-cil *

You will get with libmonogame-cil package name, version, architecture and description in a table

libmonogame-cil package basic information: