Authentication to Trello via OAuth requires the creation and registration of a custom OAuth application. This enables the account to obtain the OAuthClientId.

Do the following:

1. Visit https://trello.com/app-key.
2. Navigate to the bottom of the page.
3. Log into Trello to view your application keys.
4. Add http://localhost:33333 (or another port of your choice) to the allowed origins.