If you are using the plugin in the sandbox mode, please ensure that you are using a separate sandbox account (and not your live account) for testing, i.e. Please make sure it is not your live account but the one that is on sandbox.authorize.net. The testing guide linked below states that you must sign up for it to use the test mode:
http://developer.authorize.net/hello_world/testing_guide/
If you are using the live mode with a live account, then please try resetting the transaction key and use the newly generated one in the plugin settings page.