The easiest way would be to add stuff to the top of the screen. E.g. a heading and/or the introductory text which you suggest.
The file you need to amend is modules/profile/register.php
At the very end you'll see
case 'register':
default:
//INSERT STUFF HERE
include_once 'include/forms.php';
$reg_form =& getRegisterForm($newuser);
$reg_form->display();
break;
}
include XOOPS_ROOT_PATH.'/footer.php';
?>
Add your stuff after the line where I've put (imaginatively enough) INSERT STUFF HERE.
If you know PHP and HTML, the rest is a doddle (and the next bit will be too simple for you)
All the PHP you'd need is to insert your HTML between
echo <<<ENDOFTEXT
ENDOFTEXT;
With the HTML, it might look like:
echo <<<ENDOFTEXT
<h1>Register for this site</h1>
<p>Thank you for your interest in the site etc etc.
<p><font color="red">Your registration will be approved within 24 hours</font>
ENDOFTEXT;
I hope I've understood your question!!
EDIT: The ENDOFTEXT; shouldn't be indented, I think.