Is there a plugin for messaging users as an admin? [closed]

Yes, you can do this with the plugin you linked, Private Messages for WordPress.

Firstly, download version 2.1.09 – version 2.1.10 introduced mostly bugs, one of which excludes admins from the senders list.

Open the file pm4wp.php and edit line 450 to:

if ( current_user_can( 'administrator' ) )
{
  $users = $wpdb->get_results( "SELECT display_name FROM $wpdb->users ORDER BY display_name ASC" );
}
else
{
  $users = get_users( 'role=administrator' );
}

Look into get_users for more options.

Although as a friendly warning, you may need to do some further tweaks to make this plugin safe as it currently has a security bug. Personally I have a fix for this and the plugin author has been contacted – no response yet though.