First off if you are using 1.3.x refer to helpers via $this->HelperName->method(), there could be a variable called $media being set in some method. you can check this by doing var_dump($media);
The other option is that something has maybe unset it. Its very strange that you have the helper set but the variable is not set. It could also be due to adding the $helpers array to the wrong controller, you can try add it to app_controller and see if that works. if it does you had it in the wrong place.
If i got your second question correct, and we are talking about auto loading, a plugin controller will first look for the model in its own plugin directory, if it is not found there it will fall back to the app/models directory.
if you are loading it manually via the $uses array, it depends on the version of cake and how you do it. In previous versions 1.x even $uses = array('User'); would load the plugin model as cake would auto add the plugin prefix. This has changed for 2.0 afaik.
For other methods of loading a model, such as $this->loadModel('User); would load from app/models and $this->loadModel('PluginName.User') would load from the app/plugins/plugin_name/models dir.
Edit:
you are right that is funny having the error show $media- and there is the problem. did you copy that code from some site? − is not - you have a utf8 char in the code which is what its complaining about.
