Step 1: Create a page in your Quick Order Portal Site Admin with the name 'callback.html' - Site Admin > Site Content > Pages - click 'add new' and choose 'callback.html' from the drop down of pre-configured page names.
Step 2: In your PayPal account, there are a number of settings you must enter. Note that these settings will stop you from using this PayPal account with other shopping cart systems.