node.js - Error: Cannot find module 'webpack'

I'm just getting started with webpack and am having difficulty getting the multiple-entry-points sample to build. The webpack.config.js file in the example includes the line

 var CommonsChunkPlugin = require("../../lib/optimize/CommonsChunkPlugin");

which fails for me with the error

Error: Cannot find module '../../lib/optimize/CommonsChunkPlugin'

Searching around, I found other examples of using the CommonsChunkPlugin with the expression

var commonsPlugin = new webpack.optimize.CommonsChunkPlugin("common.js");

which fails with the error

ReferenceError: webpack is not defined

Some more searching found a number of examples including

var webpack = require('webpack');

and my build now fails with

Error: Cannot find module 'webpack'

I'm at a loss as to how to proceed.