I'm afraid I don't understand the question. Is the value populated into the hidden field?
If you want me to look at your page, please attach the actual page instead of copy/paste the code so I can open it in Dreamweaver.
Are you saying the value for: <?php echo(WA_getSavedFormValue('CheckoutWizard_NewFromBlank_Default','Last_Name')); ?> is blank?
I'd need to look at your checkout form and confirm_cart.php pages to see why.