I have just fixed the not showing up in the backend part. I changed the "limited to" condition, and had to select every product that it is limited to. If I select the category, the custom field does not show up in the backend. Which of course is weird, but since I don't have that many products, it is doable.
But the custom field is still not showing up in the order confirmation. I want to upgrade and update to the latest version. But since the last update took me very long to fix every change that it made to my website (including the sudden changes it made to the css what was exteremely annoying), I am not sure if I can update it now as I simply don't have the time.
So please provide a fix for 3.2.0, since I did pay for it.