Actually, you can add it, but you need to edit the plugin file.
If you open mailchimp_widget.php find this block of code:
<input type="text" size="4" maxlength="4" value="" name="'.esc_attr($opt.'[detail2]').'" id="'.esc_attr($opt.'-detail2').'" class="mc_input mc_phone" />
';
break;
case 'email':
case 'url':
case 'imageurl':
case 'text':
case 'number':
default:
$html .= '
<input type="text" size="18" value="'.esc_html($var['default']).'" name="'.esc_attr($opt).'" id="'.esc_attr($opt).'" class="mc_input" />';
At the end, just after class=”mc_input” add this:
placeholder="Your best email...."
Or whatever you want your placeholder to be. I’ve added it to the email field in this example, but if you search “mc_input” you’ll find options for address, name, etc.
Just keep in mind that if you update the plugin, you’ll have to go back and redo your edits.