you have to modify a bunch of different files in admin/includes/modules for it to show up in the products and categories pages. I looked at the download from the plugins section and there is a copy of admin/includes/functions/general in there under the modified core files folder. I used the version from torvista's github. I don't believe there were a lot of file changes between 1.5.5a and e. But you can see what was changed in the change log in docs/changed_files-v1-5-5.html in the zen cart install directory.