[Admin]

On the left frame, under Styles click on Modify. There click on [fonts/colors/etc] for the style you want this in.
Insert

PHP Code:

if ($bbuserinfo[userid]) { 
$registerbutton = "<a href=\"member.php?s=$session[sessionhash]&action=logout\"><img src=\"images/top_logout.gif\" alt=\"Abmelden\" border=\"0\"></a>"; 
} else { 
$registerbutton = "<a href=\"register.php?s=$session[sessionhash]&action=signup\"><img src=\"images/top_register.gif\" alt=\"Als Benutzer registrieren\" border=\"0\"></a>"; 
} 


in the field for "PHP parsed code" (By default it's

PHP Code:

// This code is PHP4 only:
// ob_start(); 
// require("yourheader.html");
// $header = ob_get_contents(); 
// ob_end_clean(); 


).

Then, write $registerbutton wherever you want the login/logout button to appear.