The sole purpose of the Processing::Proxy module is to provide access to the Processing::App variables and methods similar to the access to Inner classes afforded by vanilla processing. To do this your class should include Processing::Proxy as below:-

class MyClass
  include Processing::Proxy
  # access to sketch methods and variables is similar to java Inner class
end

Now we know this is not kosher and as you get more comfortable with JRubyArt you may eschew this egregious hack, but initially at least you will find it highly convenient. There are plenty of examples included with the samples (because it so damn convenient). See frame_of_reference sketch and Plane and Cylinder classes

The hair shirted brigade might want to take a look at forwardable and make their classes extend Forwardable instead. See the revolute_joint pbox2d example, where the Windmill and ParticleSystem both extend Forwardable.