I'm not sure what part of the instruction I am not understanding. It says I can add a mob by name, when I try I get invalid value.

That's an unfortunate limitation -- it can only ignore mobs that it already knows about, because everything is keyed off of the id. It's just that for mobs it has data on, it can look them up in the internal database to work out the id.

Let me look into this for a minute. I do something in ignoring-vignettes where I accumulate seen-so-far ones and put them into the menu for it. I can probably do a similar thing for this config.

Okay, this will now work... after you've seen the rare in that session. It'll also acquire an unchecked-checkbox in the "ignore" list for every rare you see that's not already ignored.