Thank you to AmandaGero for posting a link to an actual working site with this add-on functioning so that I could have proof that it does work. I found the problem that I am sure is what is causing everyone else headaches with getting this to function properly.

The problem is that the distribution has one file in the wrong folder.

/includes/modules/custom_template/attributes.php

should be placed into your site at this location

/includes/modules/attributes.php

My guess is that the location of this file changed in the standard distribution of Zen Cart somewhere between v1.3.7 and 1.3.8a.

I did this and got it working on the site I am building. Maybe someone could fix this in the distribution in the ZC add-on downloads so other people don't continue to have problems with this.