Thanks! One problem turned out to be that I needed to define the instructor username using a very particular case combination. Username ljensen did not work, but LJensen got me logged in. Also when defining the app in Canvas, I had to choose a "Privacy" setting to be either "Public" or "E-mail only."
With this set, the instructor can subsequently login. I haven't yet been able to check whether student accounts are correctly created in webwork after authentication through Canvas.