This one is included by default (as a dependency) with both Pattern Lab Node Core and Node Editions, so you normally won't need to install it yourself.
If it's missing from your project for any reason, npm install patternengine-node-mustache should do the trick.
Pattern Parameters and Style Modifiers are convenient syntax extensions to Mustache that allow you change the behavior of a pattern you're calling from within another. They're not necessary in other template engines because the others have natively supported ways of passing arguments to template partials.