ok, so the session is:
$_SESSION['WASF_CheckoutWizard_NewFromBlank_Default']['Country_2']
your rule should be:
Based on Session variable 'WASF_CheckoutWizard_NewFromBlank_Default' is set
AND Custom Expression:
$_SESSION['WASF_CheckoutWizard_NewFromBlank_Default']['Country_2'] != 'US'