You won’t be able to use Internet Explorer for Windows XP to access the Merchant Service Center after May 23, 2017. The good news is that any other browser (like Google Chrome) works just fine
Welcome to Intuit QuickBooks Payments
Returning User
If you have used the merchant service center before ...
First Time User
Have you previously used or do you intend to use this merchant account with QuickBooks?
 
You have indicated that you have or would like to use this account with QuickBooks.
  • If you have previously linked this account in QuickBooks, launch QuickBooks and choose Customers > Credit Card Processing Activities > Merchant Service Center (or Manage Merchant Service).
  • If you have not yet accessed this account in QuickBooks, you must first link your account in QuickBooks. Go to the Customers menu and choose Link Payment Service to Company File (or Credit Card Processing Activities > Link Merchant Account to Company File).
When you filled out your original merchant account application, did you provide an ID/Email and Password?