This is the OS I am running my mac on.

$ sw_vers -productVersion 
10.10.2

Install from https://nodejs.org. Just click on download, wait till done, click and follow the installation wizard. Once done, open the terminal and check if the versions are alright.

$ node -v 
v0.12.6
$ npm -v 
2.11.2

The installed stuff are at /usr/local/bin

$ pwd 
/usr/local/bin
$ ls -lh 
total 70416
-rwxr-xr-x  1 root  admin    34M Jul  4 06:10 node
lrwxr-xr-x  1 root  admin    38B Jul  4 22:12 npm -> ../lib/node_modules/npm/bin/npm-cli.js

Run the hello world.

var http = require('http');
http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('Hello World\n');
}).listen(1337, '127.0.0.1');
console.log('Server running at http://127.0.0.1:1337/');

References

1. https://nodejs.org 
2. http://blog.ijasoneverett.com/2013/03/getting-started-with-node-js-on-mac-os-x/