CodeKit 1.x does not have a server built-in to it and, thus, will not serve pages. In the demo videos on the website, I'm running MAMP in the background, which is set to host a given folder at http://127.0.0.1:8888/
(which maps to "localhost").
You can use MAMP or you can set up a server with Node.js, Ruby or the Apache stack built-in to OS X itself.