How can I install the grunt-cli without getting errors?

Inspired by Chris Coyier's post, I decided I'd give grunt a go. But I'm having big problems getting set up.

First, I installed Node.

Then I added a package.json file to my project root, including this:

{
  "name": "example-project",
  "version": "0.1.0",
  "devDependencies": {
    "grunt": "~0.4.1"
  }
}

Then I ran npm install

Finally, I ran npm install -g grunt-cli

which came back with a ton of errors:

npm http GET https://registry.npmjs.org/grunt-cli
npm http 304 https://registry.npmjs.org/grunt-cli
npm ERR! Error: EACCES, mkdir '/usr/local/lib/node_modules/grunt-cli'
npm ERR!  { [Error: EACCES, mkdir '/usr/local/lib/node_modules/grunt-cli']
npm ERR!   errno: 3,
npm ERR!   code: 'EACCES',
npm ERR!   path: '/usr/local/lib/node_modules/grunt-cli',
npm ERR!   fstream_type: 'Directory',
npm ERR!   fstream_path: '/usr/local/lib/node_modules/grunt-cli',
npm ERR!   fstream_class: 'DirWriter',
npm ERR!   fstream_stack: 
npm ERR!    [ '/usr/local/lib/node_modules/npm/node_modules/fstream/lib/dir-writer.js:36:23',
npm ERR!      '/usr/local/lib/node_modules/npm/node_modules/mkdirp/index.js:37:53',
npm ERR!      'Object.oncomplete (fs.js:107:15)' ] }
npm ERR! 
npm ERR! Please try running this command again as root/Administrator.

npm ERR! System Darwin 11.4.2
npm ERR! command "node" "/usr/local/bin/npm" "install" "-g" "grunt-cli"
npm ERR! cwd /Users/bryce/repo
npm ERR! node -v v0.10.23
npm ERR! npm -v 1.3.17
npm ERR! path /usr/local/lib/node_modules/grunt-cli
npm ERR! fstream_path /usr/local/lib/node_modules/grunt-cli
npm ERR! fstream_type Directory
npm ERR! fstream_class DirWriter
npm ERR! code EACCES
npm ERR! errno 3
npm ERR! stack Error: EACCES, mkdir '/usr/local/lib/node_modules/grunt-cli'
npm ERR! fstream_stack /usr/local/lib/node_modules/npm/node_modules/fstream/lib/dir-writer.js:36:23
npm ERR! fstream_stack /usr/local/lib/node_modules/npm/node_modules/mkdirp/index.js:37:53
npm ERR! fstream_stack Object.oncomplete (fs.js:107:15)
npm ERR! 
npm ERR! Additional logging details can be found in:
npm ERR!     /Users/bryce/repo/npm-debug.log
npm ERR! not ok code 0
Bryce:repo bryce$ npm install -g grunt-cli
npm http GET https://registry.npmjs.org/grunt-cli
npm http 304 https://registry.npmjs.org/grunt-cli
npm ERR! Error: EACCES, mkdir '/usr/local/lib/node_modules/grunt-cli'
npm ERR!  { [Error: EACCES, mkdir '/usr/local/lib/node_modules/grunt-cli']
npm ERR!   errno: 3,
npm ERR!   code: 'EACCES',
npm ERR!   path: '/usr/local/lib/node_modules/grunt-cli',
npm ERR!   fstream_type: 'Directory',
npm ERR!   fstream_path: '/usr/local/lib/node_modules/grunt-cli',
npm ERR!   fstream_class: 'DirWriter',
npm ERR!   fstream_stack: 
npm ERR!    [ '/usr/local/lib/node_modules/npm/node_modules/fstream/lib/dir-writer.js:36:23',
npm ERR!      '/usr/local/lib/node_modules/npm/node_modules/mkdirp/index.js:37:53',
npm ERR!      'Object.oncomplete (fs.js:107:15)' ] }
npm ERR! 
npm ERR! Please try running this command again as root/Administrator.

npm ERR! System Darwin 11.4.2
npm ERR! command "node" "/usr/local/bin/npm" "install" "-g" "grunt-cli"
npm ERR! cwd /Users/bryce/repo
npm ERR! node -v v0.10.23
npm ERR! npm -v 1.3.17
npm ERR! path /usr/local/lib/node_modules/grunt-cli
npm ERR! fstream_path /usr/local/lib/node_modules/grunt-cli
npm ERR! fstream_type Directory
npm ERR! fstream_class DirWriter
npm ERR! code EACCES
npm ERR! errno 3
npm ERR! stack Error: EACCES, mkdir '/usr/local/lib/node_modules/grunt-cli'
npm ERR! fstream_stack /usr/local/lib/node_modules/npm/node_modules/fstream/lib/dir-writer.js:36:23
npm ERR! fstream_stack /usr/local/lib/node_modules/npm/node_modules/mkdirp/index.js:37:53
npm ERR! fstream_stack Object.oncomplete (fs.js:107:15)
npm ERR! 
npm ERR! Additional logging details can be found in:
npm ERR!     /Users/bryce/repo/npm-debug.log
npm ERR! not ok code 0

I assume the above errors are the reason that when I tried to run grunt, I simply get -bash: grunt: command not found

Anyone out there know what's happening?