Hello. If your business account profile screen is the 'new' version you will see 4 tabs to the left of your screen. Click on 'My selling tools' and under 'Getting paid and managing my risk' is instant payment notifications. Click update and you should be where you need.
Also, under the four tabs is a link to the profile page clasic view. This page is just a listing of section links. In this view you would click on the 'Website Payment Preferences' link under Selling Preferences section.
Hope this helps.
