APEX giving error for name of custom object

I am a total newbie to APEX. I am using FormAssembly to integrate payments made by our volunteers to a custom object I created in SalesForce (Payments). In Salesforce, it says the name of the object for API use is "Payments_c" . However when I reference it in sample code provided by FormAssembly, I get an error that says, "sObject type 'Payments_c' is not supported. If you are attempting to use a custom object, be sure to append the '__c' after the entity name. Please reference your WSDL or the describe call for the appropriate names."

I'm sure this is very basic... I really appreciate your help. :)