Solved it, but I would suggest an edit to the Wiki so it makes more sense.
This post About prompt user input - #2 by kcwhat gave me the answer: test if Text %PromptButton% contains the button name.
@peternlewis Is that something obvious to everyone except me--that a Token should be evaluated as a Text condition rather than a variable condition? If other people have the problem, I humbly suggest editing the wiki entry on %PromptButton% to explain that, preferably with an example If action.