If the “Instant Payment Notification Preferences” link does not show up under the “Selling Preferences” column in PayPal, it means that you do not have a Business Account with PayPal.
It is easy to upgrade to a Business Account, there should be a link in your PayPal admin that says “Upgrade to a Business Account”.
Please click this link and complete the necessary information. When your account has been converted to a Business Account, the “Instant Payment Notification Preferences” link will become available to you.