If the Buy Now button-code tab is grayed-out and not clickable, that means you have product settings that cannot work with Buy Now instant checkouts.
If you can click it but no code is presented under that tab, that means you do not have any payment account specified to receive payments, such as a PayPal email in Admin > Edit Profile. If you have a PayPal email entered there, go into Admin > Payment Preferences and make sure you have checked either PayPal Standard or PayPal Website Payments Pro (if you are not sure which one you have, you almost certainly have Standard).