The command line tools are not included with the latest version of Grunt (0.4 at time of writing) instead you need to install them separately.
This is a good idea because it means you can have different versions of Grunt running on different projects but still use the nice concise
grunt command to run them.
So first install the grunt cli tools globally:
npm install -g grunt-cli
sudo npm install -g grunt-cli ).
You can establish that's working by typing
Now you can install the current version of Grunt local to your project. So from your project's location...
npm install grunt --save-dev
save-dev switch isn't strictly necessary but is a good idea because it will mark grunt in its package.json devDependencies section as a development only module.