Reading the code behind the examples in the infokit isn't very helpful to most beginners with CDF. The code is rather advanced. That said, Mathematica notebooks really are just text documents. You can open them with notepad and read them for example. Or you can look at individual cells by Cell > Show Expression in the Mathematica menu bar. Doing this isn't going to be helpful for you though. You don't want to see the code that actually runs the CDFs anyway, you really want to see the code that generates the code which runs the CDFs, which you won't find in the infokit.  That previous sentence will be confusing to most people, so don't mind it. Long story short, the infokit looks nice and shows what the CDF player is capable of, but isn't a good resource for anyone starting out. Ignore its code for now. 

If you want to see some examples of real-world CDF code in action, take a look at the Wolfram Demonstrations project. The projects are written by real customers and they come with the source code in an easy to read format. It's the same source code the customers wrote.