jaschu

I found a great answer here: http://wordpress.stackexchange.com/a/158546/58473 but I’ll try to sum up. That answer from mrwweb helped me a lot.

Not all plugins for TinyMCE are installed in the base WP codebase. You’ll have to get the anchor button javascript by downloading the full TinyMCE package ( http://www.tinymce.com/download/download.php right now ) and from the downloaded, unzipped folder, grab: js/tinymce/plugins/anchor. You could install it in /wp-includes/js/tinymce/plugins/anchor in your WP installation, but it could get overwritten when you upgrade WordPress, so either place those js files in the theme you’re using, or roll it up in a plugin.

The $buttons[] = 'anchor'; line you’ve added is correct, keep it. What’s now missing is something like this:

function my_mce_external_plugins($plugins) {
    $plugins['anchor'] = 'where-you-put-that-folder/js/tinymce/plugins/anchor/plugin.min.js';
    return $plugins;
}
add_filter('mce_external_plugins', 'my_mce_external_plugins');