Grunt - npm module not found

I'm new to Grunt and I'm having trouble setting up a project with new plugins.

So far I have:

1. installed grunt-cli globally
2. installed grunt-init globally
3. downloaded the gruntfile template from the GIT repository
4. created a new directory 'test' and navigated to the directory in Terminal
5. created a new grunt project from the template (grunt-init gruntfile)
6. installed the project dependencies (npm install)

This all works great, however I want to use a different uglify plugin so, still in the 'test' directory, I run:

npm install uglify-js --save-dev

As I'm not using the global flag, the plugin downloads to test/node_modules directory as expected, existing at test/node_modules/uglify-js. The package.json file has been updated with the additional dependency:

"uglify-js": "^2.4.15"

I then edit Gruntfile.js, adding

grunt.loadNpmTasks('uglify-js');

And change

grunt.registerTask('default', ['jshint', 'qunit', 'concat', 'uglify']);

To

grunt.registerTask('default', ['concat', 'uglify-js']);

I run

npm install

to ensure all dependencies are downloaded, but now when I run the grunt task I get this error:

Local Npm module "uglify-js" not found. Is it installed?
Warning: Task "uglify-js" not found. Use --force to continue.

Anyone have any idea what I'm doing wrong?

Mac OS X 10.8.5, Node v0.10.32