My-Meta-Box | A class for creating custom meta boxes for WordPress

You were almost there.

The final part of the puzzle is the function that saves the metadata, but first we need to make a few adjustments to your existing code:

1. add_action( 'save_post', 'cd_meta_box_add', 10, 2 ); has to be moved outside cd_meta_box_add(), and

2. Change add_action( 'save_post', 'cd_meta_box_add', 10, 2 ); into add_action( 'save_post', 'cd_meta_box_add' ); as this action hook only receives one parameter (the post ID), and

3. You need to define the function that will process the data (and it can't be cd_meta_box_add as you have it now so we'll create a new one called save_cd_meta_box_data).