I don't know about that Ruby/Gem, but your app registration is not correct.  

**You need to configure your app for explicit OAuth2, per [this other answer][1], *except* enter `localhost` instead of `example.com`.**

<sup>(The localhost is based on the original version of the question.  If you have a registered domain, use that.)</sup>

Then, in your ruby code:

    config.omniauth: stackexchange, 'CLIENT_ID', 'CLIENT SECRET', callback_url: 'http://localhost:3000/'

(Assuming the rest of that `devise.rb` code was correct.  But see [the *OmniAuth StackExchange* doc/GitHub][2] about that.)


  [1]: https://stackapps.com/a/7852/7653
  [2]: https://github.com/nashby/omniauth-stackexchange