It's really a good plugin and the fact that it vanishes after you've signed up was the deal breaker for me on choosing this plugin. However, I would give it five stars if it were easier to visually modify. There's no sub header. Rather than having the admin enter the error message, write some better error messaging into the widget to validate the names and email address.
Also, your mcapi.class.php file looks like it's for some other project. Maybe not force us to inhale that file if it's not necessary.