Really fundamental problem

I can't seem to get past the simple task of selecting files. I assign a folder that contains several PDF files, use a selection criterion such as Kind is PDF, and an action such as add a green tag, and nothing happens. When I click on the eye icon, no files are shown to have matched.

I may have fundamentally misunderstood how to specify files. I may have installed Hazel incorrectly. I bought the MacSparky video field guide and I thought I was following the file description as was demonstrated in that video, but have been unable to make Hazel recognize files meeting the specified criterion.

Can you post the specific rules you are using? Also, have you tried using the preview function when editing your rules? Check out this article for more details: https://www.noodlesoft.com/kb/previewing-your-rule/