Launching an ubuntu instance on EC2 does not automatically make it a server. You need to actually run a web-server in order to see files from that computer on your browser.
For static files, you can use simple web server like python's SimpleHTTPServer or webfsd.
If you are planning to work with node.js, you might prefer writing a small Hello World in node.js instead:
// Load the http module to create an http server.
var http = require('http');
// Configure our HTTP server to respond with Hello World to all requests.
var server = http.createServer(function (request, response) {
response.writeHead(200, {"Content-Type": "text/plain"});
response.end("Hello World\n");
});
// Listen on port 8000, IP defaults to 127.0.0.1
server.listen(8000);
// Put a friendly message on the terminal
console.log("Server running at http://127.0.0.1:8000/");