Connecting Node/Mongo app to Heroku/MongoLab

Trying to do what the title says.

When I try to heroku open I get Error: failed to connect to [localhost:27017]. I know this is because I don't have any database hooked up to my Heroku app. What I don't know is how to do that.

One tutorial said this snippet would work:

var mongo = require('mongodb');

var mongoUri = process.env.MONGOLAB_URI ||
  process.env.MONGOHQ_URL ||
  'mongodb://localhost/mydb';

mongo.Db.connect(mongoUri, function (err, db) {
  db.collection('mydocs', function(er, collection) {
    collection.insert({'mykey': 'myvalue'}, {safe: true}, function(er,rs) {
    });
  });
});

But I'm not sure where exactly to put it (app.js?) or what mykey and myvalue are. I see MongoLab has a unique username and account name, but I can't find anything else. Where should I look/do to fix this problem?