This looks like you are using test mode and are getting the message separately. For the id you should be using the primary login ID for your Authorize.Net account, which it sounds like you are.
I think your account may just not be fully setup yet, if you want to try and toggle the test mode this can be done by editing the server behavior for the local checkout on the confirm page, or by editing the code and finding the test setting and making it false.
I think if you were using the wrong credentials you would get a different error message but I have not configured an Authorize.NET account myself.