Intuit has changed the Intuit Payment Network Service for 2015 and newer versions and you may experience issues when trying to setup the Payment or Merchant Services in QuickBooks after upgrading from 2014.
When going to Customers>Link Payment Service to Company File, you should be prompted to login to their MS account however you may reach a "Page Cannot be Displayed" error.
Intuit has required that anybody moving from a previous year of QB will need to first call Merchant Services and have their account un-linked from their previous QB file. You will need to call Intuit to perform this function. Once it has been un-linked, then you will need to go to payment.intuit.com and set up a new account with a fresh/new email address.
Once you obtain this, you can go to Customers>Link Payment Service to Company File and link your new service to your upgraded QuickBooks file.