$ sudo apt-get update
$ sudo apt-get install monodevelop-nunit 

monodevelop-nunit is installed in your system.

Make ensure the monodevelop-nunit package were installed using the commands given below,

$ sudo dpkg-query -l | grep monodevelop-nunit *

You will get with monodevelop-nunit package name, version, architecture and description in a table

monodevelop-nunit package basic information: