Paypal WPP (US) not working for OC1.4.0

Hi,

I have installed PayPal Website Payments Pro in Opencart 1.4.0. I have given Test API credentials in admin panel.
But it is not working. When I put credit card details and confirm the order, it shows only "Please wait!" message.
Normal PayPal is working fine.

Please help.

Thanks
Amit