To start accepting new types of credit cards like American Express or Discover, follow these steps:

1. Reach out to your Merchant Service Provider (MSP). The MSP operates as your financial institution, managing the deposit of funds into your bank account and maintaining your merchant account with Visa and MasterCard. They also charge you the percentage discount rate on your batch settlements.
2. Your MSP arranges and updates your payment processor to allow you to accept the new card type.
3. Once your MSP has updated your merchant account and the payment processor, modify the credit card types you are accepting within your Authorize.Net account:
   1. Log into the Merchant Interface.
   2. Select Account from the main toolbar.
   3. Select Merchant Profile from the main menu.
   4. Click the View Configuration link on the right-hand side under the Payment Methods section.
   5. Click the Edit button on the right-hand side of the Payment Types Accepted section.
   6. Add or remove the credit card types as needed by checking or unchecking the boxes next to each payment type.
   7. Click Save near the bottom of the page.
   8. Request and enter PIN for verification.
   9. Confirm that your payment types have been updated successfully.

Remember, by enabling Discover Card acceptance, you automatically enable the acceptance of card brands like Diners, UnionPay, and JCB. Therefore, Diners and JCB should be enabled as accepted card types on your Authorize.net account.

If you don't see the option for View Configuration and instead see Multiple processors, and/or if you have eCheck.Net or PayPal enabled, contact Authorize.net support for help enabling/disabling card types for your account. You can reach them via phone, chat, or a Support case.