Ok, I tested through your website and this is just coming from the fact that these fields are required and if you register through the joomla form, these fields won't be filled.
But I didn't had any error message and managed to successfully go through the paypal payment platform.
Can you show me how did you set your checkout workflow through some screenshots ("Hikashop->System->Configuration->Checkout").
PS: Regarding your login/password field, using some CSS code like this will probably make them look better :
#passwd, #username {
width: 166px;
}