it says right in the instructions that you just include the class
//include the main class file
require_once("meta-box-class/my-meta-box-class.php");
you’d put that in whatever file you are using for the code blocks you posted… not in the function you’ve written.
looks like there is a pretty good section called “Usage” at the link you provided. you should re-read that and revise your question to explain more clearly what your problem is that is not answered by the provided documentation.