I have installed the MJML from node package manager globally and then created a mjml filetype in PhpStorm and added a new File Watcher for MJML files with the following settings.
Type: MJML from the dropdown
Argument: --watch $FileName$ -o $FileNameWithoutExtension$.html
Working Directory: $FileDir$
Build #PS-163.10504.2, built on December 20, 2016
Licensed to PhpStorm Evaluator
Expiration date: April 3, 2017
JRE: 1.8.0_112-release-408-b6 amd64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
For some reasons it does not watch and generate the output file.