merlin is an object-oriented, MVC framework for node.js and CouchDB. It's built on top of Express and Cradle. Its name comes from Drupal, under which the author has slaved for far too long.
Firstly, you should understand Model-View-Control architecture in order to use merlin. If you don't yet, you might want to read up on it somewhere a little more verbose than here.
Now, before building out your classes, you'll need to load up merlin and whichever of his plugins you think you might need.
var merlin = require('merlin').initMerlin(
// global merlin settings come in the first argument