Can’t call external stylesheet for WordPress admin (using wp_admin_css)?

Question

You need to use wp_enqueue_style and hook it into admin_print_styles

add_action( 'admin_print_styles' , 'my_wp_admin_css' );

function my_wp_admin_css() {

      wp_enqueue_style('my_admin_style' , WP_PLUGIN_URL . '/myPlugin/stylesheet.css' );
}

If you just want the stylesheet on your plugins page you can use:

$mypage = add_management_page( 'myplugin', 'myplugin', 9, __FILE__, 'myplugin_admin_page' );
add_action( "admin_print_styles-$mypage", 'myplugin_admin_head' );
 
function myplugin_admin_head() {
    // what your plugin needs in its <head>
}