Okay, so I tried this on the SDK and the online api 'try it' sections... i changed the cardCode = "xxx" and i get a response. however on what im trying to implement i get a null for the response. I have attached parts of my code and the parameters im passing...I am only trying to authorize a transaction. 

Parameters-

{
String ApiLoginID = WebServices.getPreference("authorizenetLoginId");
String ApiTransactionKey =WebServices.getPreference("authorizenetTransKey");
string cc = "4111111111111111";
string expm = "11";
string expy = "16";
string cvv2 = "xxxx";
string order = "Towels";
decimal amount = Decimal.Parse("12345.65789");
string summary = string.Concat(order,"-",amount);
string name = "doggy dog cat";
string addr1 = "Testaddr1";
string addr2 = "Testaddr2";
string city = "Testcity";
string state = "teststate";
string zip = "testzip";
string country = "testCountry";
string email = "Test@email.com";
string phone = "1111117777";
string shipaddr1 = "TestShipaddr1";
string shipaddr2 = "TestShipaddr1";
string shipcity = "Testshipcity";
string shipstate = "testshipstate";
string shipzip = "testzipship";
string shipcountry = "testcountryship";
AuthorizeNetHelper.AuthorizeTransaction(ApiLoginID, ApiTransactionKey, cc,expm,expy,cvv2,order,amount,summary,name,addr1,addr2,city,state,zip,country,email,phone,shipaddr1,shipaddr2,shipcity,shipstate,shipzip,shipcountry);

authorizeNet response-

public static ANetApiResponse AuthorizeTransaction(String ApiLoginID, String ApiTransactionKey, string cc, string expm, string expy, string cvv2, string order, decimal amount, string summary, string name, string addr1, string addr2, string city, string state, string zip, string country, string email, string phone, string shipaddr1, string shipaddr2, string shipcity, string shipstate, string shipzip, string shipcountry)
{
ApiOperationBase<ANetApiRequest, ANetApiResponse>.RunEnvironment = AuthorizeNet.Environment.SANDBOX;

// define the merchant information (authentication / transaction id)
ApiOperationBase<ANetApiRequest, ANetApiResponse>.MerchantAuthentication = new merchantAuthenticationType()
{
name = ApiLoginID,
ItemElementName = ItemChoiceType.transactionKey,
Item = ApiTransactionKey,
};
// Credit Card information to be used for transaction
var cCard = new creditCardType
{
cardNumber = cc,
expirationDate = string.Concat(expm, expy),
cardCode = cvv2
};
//Billing Information to be used for transaction
var billingAddress = new customerAddressType
{
//not sure what to do about fName and lName, we only have "name" in DB, if we leave the name as firstName,
//6It could work but possible problems in finding by name?
firstName = name,
//lastName = "Doe",
address = addr1,
//address2?????
city = city,
state = state,
zip = zip,
email = email,
phoneNumber = phone,
country = country
};
//Shipping Information to be used for transaction
var shippingAddress = new nameAndAddressType
{
firstName = name,
address = shipaddr1,
//Shipaddress2
city = shipcity,
state = shipstate,
zip = shipzip,
country = shipcountry
};

//Order Information for the order
var orderInfo = new orderType
{
invoiceNumber = order,
description = summary
};

//standard api call to retrieve response
var paymentType = new paymentType
{
Item = cCard
};

// // Add line Items
// //var lineItems = new lineItemType[2];
// //lineItems[0] = new lineItemType { itemId = "1", name = "t-shirt", quantity = 2, unitPrice = new Decimal(15.00) };
// //lineItems[1] = new lineItemType { itemId = "2", name = "snowboard", quantity = 1, unitPrice = new Decimal(450.00) };

//Build the Transaction
var transaction = new transactionRequestType
{
transactionType = transactionTypeEnum.authOnlyTransaction.ToString(), // Authorize amount on the card
amount = amount,
billTo = billingAddress,
order = orderInfo,
payment = paymentType,
shipTo = shippingAddress

};
//Make the transaction Request
var request = new createTransactionRequest { transactionRequest = transaction };

// instantiate the contoller that will call the service
var controller = new createTransactionController(request);

controller.Execute();
// get the response from the service (errors contained if any)
var response = controller.GetApiResponse();