npm install -g less does not work: EACCES: permission denied

I’m trying to set up less on phpstorm so I can compile .less files to .css on save. I have installed node.js and the next step (according to this https://www.jetbrains.com/webstorm/help/transpiling-sass-less-and-scss-to-css.html) is running this command in the terminal

npm install -g less

However when I do this I get these errors

☁  ~  npm install -g less
npm WARN install Couldn't install optional dependency: EACCES: permission denied, mkdir '/Users/brentscholl/.npm/mkdirp/0.5.1'
npm WARN install Couldn't install optional dependency: EACCES: permission denied, mkdir '/Users/brentscholl/.npm/graceful-fs/3.0.8'
npm WARN install Couldn't install optional dependency: EACCES: permission denied, mkdir '/Users/brentscholl/.npm/extend/3.0.0'
npm WARN install Couldn't install optional dependency: EACCES: permission denied, mkdir '/Users/brentscholl/.npm/readable-stream/2.0.4'
npm WARN install Couldn't install optional dependency: EACCES: permission denied, mkdir '/Users/brentscholl/.npm/chalk/1.1.1'
npm WARN install Couldn't install optional dependency: EACCES: permission denied, mkdir '/Users/brentscholl/.npm/xtend/4.0.1'
npm WARN checkPermissions Missing write access to /usr/local/lib/node_modules
/usr/local/lib
└─┬ less@2.5.3
  ├─┬ errno@0.1.4
  │ └── prr@0.0.0
  ├── image-size@0.3.5
  ├── mime@1.3.4
  ├─┬ promise@6.1.0
  │ └── asap@1.0.0
  └─┬ source-map@0.4.4
    └── amdefine@1.0.0

npm ERR! Darwin 15.0.0
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "install" "-g" "less"
npm ERR! node v5.0.0
npm ERR! npm  v3.3.6
npm ERR! path /usr/local/lib/node_modules
npm ERR! code EACCES
npm ERR! errno -13
npm ERR! syscall access

npm ERR! Error: EACCES: permission denied, access '/usr/local/lib/node_modules'
npm ERR!     at Error (native)
npm ERR!  { [Error: EACCES: permission denied, access '/usr/local/lib/node_modules']
npm ERR!   errno: -13,
npm ERR!   code: 'EACCES',
npm ERR!   syscall: 'access',
npm ERR!   path: '/usr/local/lib/node_modules' }
npm ERR!
npm ERR! Please try running this command again as root/Administrator.

npm ERR! Please include the following file with any support request:
npm ERR!     /Users/brentscholl/npm-debug.log

I’m a complete noob here and not sure what to do next. Any help would be very appreciated!