The best thing to do is modify the header template of your forum's theme and add the menu bar, for example: http://openfantasy.org/forum/
If you want to avoid doing that for whatever reason, I suggest you should add a redirect in phpBB to the page you want post-login.
Look at lines 76-81 in root\ucp.php (phpBB 3.0.11)
if ($user->data['is_registered'])
{
redirect(append_sid("{$phpbb_root_path}index.$phpEx"));
}
login_box(request_var('redirect', "index.$phpEx"));
change that to the URL of the page with the IFrame, and you should be good.