After you've installed and enabled the Add PHP mambot from:
www.run-digital.com/download/mambots.html
You can paste the line :
{rdaddphp file=administrator/components/com_404_redirect/404_redirect.php}
Into your 404 file in Admin > Artio JoomSef > Artio JoomSef Configuration > Default 404 Page
Paste the line of code in the WYSIWYG editor, it should say something like "404 / Page Not Found ...etc..."
Put the mambot code line in that box and save the config settings.
that should do it,
cheers,
DT