@tristanslater
The quick and dirty way to add placeholder text is to edit the plugin file.
Edit this file: /wp-content/plugins/checkmail-validation-for-contact-form-7/checkmail-validation-for-contact-form-7.php
Around Line 117 change this:
$html = '<input type="text" name="checkmail_' . $name . '" />';
to this:
$html = '<input type="text" name="checkmail_' . $name . '" value=""' . $atts . ' placeholder="Confirm Email Address" />';
So you’re basically just manually adding the placeholder text to the text field.
Also remember that if the developer ever updates the plugin, your edit to the checkmail-validation-for-contact-form-7.php file will be overwritten.