Hello,
If you want to change the styles you need to play with custom CSS and form code.
For example, to get the form like on our site you may use our code http://xstore.helpscoutdocs.com/article/66-mailchimp-form-custom-styles
and then also add the custom styles to display button in the way you like http://prntscr.com/hjgtu7
More information how to customize contact form you can find in plugin documentation https://wordpress.org/plugins/mailchimp-for-wp/ http://prntscr.com/hjgwt1
Regards