[Andreas]

First of all, you should specify a default action:

PHP Code:

if (!$_REQUEST['do']) {
  $_REQUEST['do'] = 'update';
} 


Furtheremore, vB3 (this is for Vb3, right?) does always use do= for actions, so you should also use this:

PHP Code:

if ($_REQUEST['do'] == "update")
{ 


Then, if this hack is not just for yourself but for release you should keep in mind ppl using table prefixes:

PHP Code:

$excludedforums=$DB_site->query_first("SELECT excludedforums FROM" . TABLE_PREFIX . "topx_admin WHERE topx_adminid = '1'"); 


You must also use the same column name as in your table when you use print_input_row:

PHP Code:

print_input_row("Forums to exclude", 'exclude_forums', $excludedforums['excludedforums']); 


As already stated above, you must always quote strings from a form or URL, as otherwise this will be a high security risk:

PHP Code:

$DB_site->query("UPDATE " . TABLE_PREFIX . "topx_admin SET excludedforums='" . addslashes($_POST['exclude_forums']) . "' WHERE topx_adminid = '1'"); 


PHP Code:

define('CP_REDIRECT', 'topxadmin.php?do=update'); 


Once again, you should use do= here.

As a last thing, your script is missing print_cp_footer();

Here is the whole script (topxadmin.php in admincp directory):

PHP Code:

<?php

// ######################### ERROR REPORTING #############################
error_reporting(E_ALL & ~E_NOTICE);

// #################### DEFINE IMPORTANT CONSTANTS #######################
define('NO_REGISTER_GLOBALS', 1);
define('GET_EDIT_TEMPLATES', true);
define('THIS_SCRIPT', 'topxadmin');

require_once('./global.php');
require_once('./includes/functions_misc.php');

print_cp_header('Top X Admin');

if (!$_REQUEST['do']) {
  $_REQUEST['do'] = 'update';
}

// ###################### Edit Settings ########################
if ($_REQUEST['do'] == "update")
{
  $excludedforums = $DB_site->query_first("SELECT excludedforums FROM " . TABLE_PREFIX . "topx_admin WHERE topx_adminid = '1'");

  print_form_header('topxadmin', 'do_update_topxsetting');
  print_table_header("Top X Settings");
  print_input_row("Forums to exclude", 'exclude_forums', $excludedforums['excludedforums']);
  print_submit_row("Update Top X Settings", 0);
}

if ($_POST['do'] == "do_update_topxsetting")
{
  $DB_site->query("UPDATE " . TABLE_PREFIX . "topx_admin SET excludedforums='" . addslashes($_POST['exclude_forums']) . "' WHERE topx_adminid = '1'");

  define('CP_REDIRECT', 'topxadmin.php?do=update');
  print_stop_message('excluded_forums_updated');
}

print_cp_footer();

?>

This does work for me, given your table looks like that:
[sql]
CREATE TABLE `topx_admin` (
`topx_adminid` int(10) unsigned NOT NULL auto_increment,
`excludedforums` varchar(255) NOT NULL default '',
PRIMARY KEY (`topx_adminid`)
) TYPE=MyISAM AUTO_INCREMENT;
[/sql]