[Stuck]#NodeJS - Callback issue?

Nice tutorial here : http://net.tutsplus.com/tutorials/javascript-ajax/introduction-to-express/
However, I get issues in turning static to dynamic part :(

/Users/txshon/Dropbox/Google Drive/04)Practice/NodeJS/TutExpress/nodemodules/express/lib/application.js:174
if ('function' != typeof fn) throw new Error('callback function required');
^
Error: callback function required
at Function.app.engine (/Users/txshon/Dropbox/Google Drive/04)Practice/NodeJS/TutExpress/nodemodules/express/lib/application.js:174:38)
at Object.<anonymous> (/Users/txshon/Dropbox/Google Drive/04)Practice/NodeJS/TutExpress/app.js:18:5)
at Module.compile (module.js:456:26)
at Object.Module.extensions..js (module.js:474:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Function.Module.runMain (module.js:497:10)
at startup (node.js:119:16)
at node.js:901:3