you already have the validation i nplace to check if the email exists, to show the error box, create it on your page, then select it and use the validation Show If Behaivor arournd it so it will only show if the Numeric validation fails.
for the success box, the insert creates a session variables named:
pcms2_Newsletter_UserID
use an if statement to check if that session is set to show the success message:
<?php if(isset($_SESSION['pcms2_Newsletter_UserID']) && $_SESSION['pcms2_Newsletter_UserID'] != "") { ?>
Success message here
<?php } ?>
and add the following code at the bottom of the page to reset the session:
<?php if(isset($_SESSION['pcms2_Newsletter_UserID'])) unset($_SESSION['pcms2_Newsletter_UserID']); ?>