Mjform.com

Hello Joomlanians, I've spent hours and hours and hours now searching the Joomla Docs, Google and forums and trying stuff. I hope someone here can help me. I'd like to display a custom field I called "walls" in the Article Category Module. In the screenshot I highlighted what I added to an override of the standard default.php file in html/mod_articles_category. I'm not a php pro, but I usually can find my way around it and in my understanding this should work. What am I missing?