how to install mpkg file

Weird!

Right-click (or ctrl-click) on it and choose 'open with' and see what applications are presented. If it's there, choose 'Installer'.

If that fails, right-click again and choose 'show package contents'. Post the list here if you can't see what might be the problem.

You should be able to select the file and open it. From there it will go through an install process.

Double-click the .mpkg file.