OK, I set up the AN module with a valid user and transaction key (after realizing that I had to put in the joomla admin pass TWICE to get in there.) I have it in testing mode=yes I assume that directs to the auth net testing or staging server. I have it configured to use payment processor and to accept several credit cards visa of course. So I get in the catalog, log in as a user, select a product, go through the whole purchase chain and for credit card I insert the testing number 4111-1111-1111-1111 and some bogus CVV and user name etc. The message I get back is this:
Warning: Sorry, but the Credit Card Number you've used is a testing number!
Umm.... yeah I really don't want to test on my real cc number, this product costs $5000!
HOW DO YOU TEST IT IF YOU CANT USE 411?
What I would do is create and setup a test product with a lower amount. Then purchase that and see if the transaction goes through. Don't forget to unpublish or delete the test product after your done.
You register for a developer key, use a real credit card and wala, it works. It won't bill your real card and will show in your system as successfully paid