wp_dashboard_setup doesn't work, what's wrong ?

Hi,
I’m kinda new to WordPress …. I’m actually trying to create a basic bashboard widget but noting works, neither directly in functions.php, neither whithin a plugin file, what am i doign wrong ?

add_action( 'wp_dashboard_setup', 'add_dashboard_widget' );
function add_dashboard_widget(){
	wp_add_dashboard_widget( 'hello_widget', 'Hello', 'hello_widget' );
}
function hello_widget(){
	echo "something to display";
}

Thank’s