Confused over custom fields

I’m rather confused over the way to add custom fields to the cart. I first added the code described here:

http://docs.joedolson.com/my-tickets/2014/11/25/custom-field-api/

I stuck it in the theme’s function.php just to get it working. But nothing seemed to happen. But then I came across another post which directed me to here:

https://github.com/joedolson/plugin-extensions

I’ve created the custom plugin and the “Code of conduct” field does indeed appear on the cart *but* the receipt has a reference to meal preference.

Do the two systems work together?