That demo plugin actually does not contain the necessary filters to add to support for the content box. I’ll try to remember to add them so the demo is more complete. When I made the custom fields demo, the content block support didn’t exist.
You might want to check out the Education Level or Income Level extensions instead. They have all the required filters and are pretty well documented.
It odd you had to add that line of code to Slim Plus, it should already exist on line 63 in the card.php file.
Hope that is helpful.