503998 authorizenet gateway form needs to be configured

I have an existing check out registration form where I can edit the API username and password, signature and email and id from my personal admin control panel. it was configured to work with paypal so after the person selected there products it would bring the amount owed to paypal for checkout. I now have [login to view URL] and I need for my form to be configured process the payment using [login to view URL] secure server. i need this completed within 1 day or a full refund will be provided.