Hooking does not seem to work?

Probably my own fault somehow, but if somebody could take 2 minutes and help me with this it would be awesome:

I more or less copied an example for a hook into the functions.php of my theme.

add_action( 'init', 'ninja_forms_register_example' );
	function ninja_forms_register_example(){
		add_action('ninja_forms_post_process', 'ninja_forms_example');
	}

	function ninja_forms_example(){
		global $ninja_forms_processing;

		//Add the text Hello World to the user value.
		$user_value = "Hello World.";

		//Update the user submitted value for the field with an ID of 11:
		$ninja_forms_processing->update_field_value( 11, $user_value );
		$ninja_forms_processing->update_field_value( 2, 1 );

	}

The add_action seems to work, i put an echo command into the ninja_forms_register_example and it shows up on the page. However the second part does not work. Field 11 is a text field, so it should work. Field 2 is a radio button with 5 choices, so value 1 should be valid as well?! I can also change ninja_forms_post_process for other hooks but it doesn’t change the outcome, sometimes the “Processing..” will just show up and nothing will happen further.

What am i doing wrong?

http://wordpress.org/extend/plugins/ninja-forms/