Problème avec script PHP

Bonjour,
j'ai télécharger un script de e-comerce depuis ce site:
http://www6.phpwebcommerce.com/?tdfs=1&kw=Ecommerce+Software&term=Ecommerce%20Shopping%20Cart%20Platform&term=Ecommerce%20Online%20Store&term=Digital%20Marketing%20Services&backfill=0
cependant j'ai des soucis pour me connecter à la partie admin: voici le script qui gére la connexion:
___________________________________________
function checkUser()
{
// if the session id is not set, redirect to login page
if (!isset($_SESSION['plaincart_user_id'])) {
header('Location: ' . WEB_ROOT . 'admin/login.php');
exit;
}

// the user want to logout
if (isset($_GET['logout'])) {
doLogout();
}
}

/*

*/
function doLogin()
{
// if we found an error save the error message in this variable
$errorMessage = '';

$userName = $_POST['txtUserName'];
$password = $_POST['txtPassword'];

// first, make sure the username & password are not empty
if ($userName == '') {
$errorMessage = 'You must enter your username';
} else if ($password == '') {
$errorMessage = 'You must enter the password';
} else {
// check the database and see if the username and password combo do match
$sql = "SELECT user_id
FROM tbl_user
WHERE user_name = '$userName' AND user_password = PASSWORD('$password')";
$result = dbQuery($sql);

if (dbNumRows($result) == 1) {
$row = dbFetchAssoc($result);
$_SESSION['plaincart_user_id'] = $row['user_id'];

// log the time when the user last login
$sql = "UPDATE tbl_user
SET user_last_login = NOW()
WHERE user_id = '{$row['user_id']}'";
dbQuery($sql);

// now that the user is verified we move on to the next page
// if the user had been in the admin pages before we move to
// the last page visited
if (isset($_SESSION['login_return_url'])) {
header('Location: ' . $_SESSION['login_return_url']);
exit;
} else {
header('Location: index.php');
exit;
}
} else {
$errorMessage = 'Wrong username or password';
}

}

return $errorMessage;
}
___________________________________________

j'aimerai savoir où se trouve le souci, ou bien comment contournercette page. J'ai bien vérifier la base données avec les identifiants : admin - admin.

merci