Logout Link

I have installed the "Login" script on my blog, but when the user logs in, it does not show that they are logged in nor does it show the logout button. Am I doing something wrong?

Here's the code as it appears on the site:
<!-- login box start -->
<div id="login_box">
<?php
if ($au=$_SESSION['_amember_user']){ // user is logged-in
print "Hello $au[name_f] $au[name_l]!<br>";
print "<a href='/amember/logout.php'>Logout</a>";
} else { // user is not logged-in
print "<form method=post action='/amember/login.php'>
<input type=text name=amember_login value=username size=10>
<input type=password name=amember_pass value=password size=10>
<input type=submit value='Login'>
</form>";
}
?>
</div>


<!-- login box end -->