I'm seeing a 500 there, it seems as your server can't connect to Paypal, does your hosting provider allows you to create sockets? Ask them if you can run a line like this to contact other servers from yours:

$fp = fsockopen ($paypal_host, 80, $errno, $errstr, 30);

Are you doing this in Paypal's Sandbox? Did you correctly create an account there?

Alexis Bellido