You can modify this file right where it's at, then use the compiling tools on the data source's page to compile your changes and have it appear in the templates list (all without having to restart Mango). You may have to switch template types and back again after the compiling status has returned to have the template in the select list.

You may also find it easier to use

private SimpleDateFormat dtf = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");

instead of the DateTimeFormatter in the MulticolumnCSVImporter example.

I'm not very familiar Intellij IDEA but our projects are maven projects now, so you could try its Maven plugin if you find you really do need a full environment.