In order to activate a credit card in MineralTree, the Account in NetSuite must be of the "credit card" type.
To do this, navigate to Lists > Accounting > Accounts > New
Then enter the credit card information and save.
In this example, the account number is in the format of a full credit card number, but that is not necessary for success and any normal account number that will fit with your Chart of Accounts structure will suffice.
Please note: Subaccounts must have the same type as accounts, so a Credit Card account cannot be the subaccount of a Bank account.