never mind, I managed to work for me:
wp_register_script( 'teo_dd', plugins_url( '/msdropdown/jquerydd.min.js'), array( 'jquery-core' ), '20140608', true );
wp_enqueue_script( 'teo_dd' );
without using a function and add_action():
function add_teo_dd(){
wp_register_script( 'teo_dd', plugins_url( '/msdropdown/jquerydd.min.js'), array( 'jquery-core' ), '20140608', true );
wp_enqueue_script( 'teo_dd' );
}
add_action('wp_enqueue_scripts', 'add_teo_dd');