No need to create a plugin.
You just have to put that piece of code in "functions.php" file inside your WordPress Theme.
function mt_add_pages() {
add_menu_page('admin-menu', 'menu-name', 5, __FILE__, 'mt_toplevel_page');
}
function mt_toplevel_page() {
echo '
<div class="wrap">
<h2>New admin menu</h2>
<li><a href="http://www.icore.net.tc"><h3>Author Homepage</h3></a></li>
</div>';
}
add_action('admin_menu', 'mt_add_pages');
Complement this with the CODEX info, and you will crete custom admin menu's for your themes. And if we helped you, please, change this post to [resolved]. Bye.