having problem running a test transaction with authorize.net


Can you please explain how to properly run a test transaction using market press with authorize.net. I have authorize.net in test mode and market press in sandbox, when I try to run a transaction using 4222222222222 as the credit card I get an error "There was a problem finalizing your purchase. (TESTMODE) The merchant login ID or password is invalid or the account is inactive." I've double checked the merchant ID and it is correct. If I take authorize.net out of test mode and switch market press to live and try I get the message "The credit card number is invalid". How do I go about testing a transaction? Or do I have something setup wrong?