Digger, to add a plant to your list, you can select the Blue, "Add this plant to your list" button on a CVs page. It is located on the left side of the screen. See photo.
This will allow a pop up to show on your screen and then you can select the options.
The photo above reflects categories I created, which is another discussion. You should see a general screen to add. Unfortunately, I don't have a photo of the general screen.
Edit to add...
I am sure there is a topic for creating your categories, but on your list page, you can select manage categories.
Also, here is the link to help page.
https://garden.org/help/