Rails admin is an awesome tool. But when it comes to administer models that are not defined in your application, but in an engine, they simply don't show up.
But....if you manually add them to the model list in an initializer config file, they show up. Assume you have an admin model in your app and a news model inside an engine.
RailsAdmin.config do |config| config.included_models = [Admin, News] end
Now both show up. The only caveat is, that you'll have to manually add all models.