admin_enqueue_scripts

plugin.php

<?php

/**
 *
 * admin_enqueue_scripts is the preferred hook[1] for adding style to the admin, but
 * documentation is frankly sparse and a bit confusing, especially for beginners.
 *
 * This gist has everything you need to load style for a plugin directly to a hook
 * of your choice. Enjoy!
 *
 * [1] http://bit.ly/vYw3SY
 *
 */

function plugin_load_style($hook) {
	if ( $hook == 'your_hook_name' ) {
		wp_register_style('plugin_style', plugins_url('css/style.css', __FILE__));
		wp_enqueue_style('plugin_style');
	}
}
add_action('admin_enqueue_scripts', 'plugin_load_style');