If you google for node.js, the first result shows the homepage, there is ab big "install" button (takes about 3-4 seconds to find that) ;)
However, I would take a look for "homebrew" if you have to install some "custom" stuff on the mac (node.js, databases, ...). Pretty nice tool (after that you can install node.js by simply typing "brew install node". Great for updating stuff too.
And as you won't stop with node.js and "a testing tool", take a look at some bower tutorial (to manage JS includes/frameworks).