How do I access the Node Access User Reference configuration?
I found a clue in the README.txt which I'd previously missed


  Create a field to reference users in a content type using the Field UI.  The field's
  configuration page will contain the settings for Node access user reference.


I edited the User Reference field I'd created, and on the Edit tab was a collapsed heading for Node access user reference that I'd previously missed.  Clicking the heading revealed the configuration form.