No, it's not at all obvious. When you click into the "Muted" box, it will show a highlighted box, and when you start to type in there, it will display a dropdown menu of suggested categories for what you've typed, and you can choose from there. Repeat as necessary.
To remove them again, you just click on the "x".
Again, I agree.
The moderators have been testing the software for months, and there are still things I don't know how to do, or have only recently discovered.