見這裏：http://stackoverflow.com/questions/11663590/issues-with-installing-express-js-in-windows-7?rq=1
In later versions of express comand line was migrated to a separate module: express-generetor
說的是最新版本的express，命令行功能已經轉移到模塊express-generetor，需要單獨安裝．
使用如下命令
npm install -g express-generator
