Those variables in cc_class.php should be the only files that need to be modified. If you were having an authentication issue though, it would be returning a 401 error and you would still be able to see the form elements on the page.
Can you copy/paste exactly the error message you are seeing on your screen?
Are there any other warnings or errors that are on this page other than the 500?
Would you be able to give us the URL to the page that is having this error?
View API documentation, code samples, get your API key.