Symfony2admingenerator changed the way you can provide Entity/Model parameters to your custom route

gistfile1.yml

builders:
  list:
    params:
      title: List of elements
      display: [a, b, c]
      actions:
        new: ~
      object_actions:
        your_custom_action:
          route: your_custom_route
          params:
            pk: '{{YourModel.id}}' # Notice that curly braces are quoted, allowing parsing in the generated twig template
        edit: ~
        delete: ~