I wanted to add a lil note, but i see that there is no edit button ???

anyways, this goes under the last Code Box...
this assumes the following:
you are using a zen_ table heading if you are not then you need to remove all occurrences of zen_ or replace them with your own table heading.
also $UserID should be gained by the authentication process which you should paste this into in order for things to work.

however you have things set up, you have to make sure that your custumer id comes from the zencart, in other words:
Code:
$mysql ="SELECT customers_id, customers_password FROM zen_customers WHERE customers_email_address='".$_POST['email-address-provided-by-custumer-as-login']."' LIMIT 1";
$result = mysql_query($mysql [, $connection_link]) or die ("most likey the user does not exist in this case..." . mysql_error());
$row = mysql_fetch_array($result);

$UserID = $row['customers_id'];
$pass = $row['customers_password'];

// now compare passwords and autheninticate the user and you can also perform the log in...
MODERATOR NOTE: The code suggested here is vulnerable to SQL Injection attack, since the input to the database query is not sanitized. USE AT OWN RISK