How to use log.io logging module in Node.js

In my node application i want to use log.io logging module to store my application log messages.Now i have installed that module using this docs.https://github.com/NarrativeScience/Log.io Now i dont know which method in log.io have to call to store my log.

This is my app file.

 var express=require('express');
 var app=express();
 app.use(express.static(__dirname + '/public'));
 app.listen(9090);
 var logger = require('log.io');
 logger.whichMethodToLogMsg('my error msg');