I was having the "There has been an error processing your credit card. Please try again and if problems persist, please try another payment method." error message with this module as well. After digging through hundreds of threads on this topic I finally was able to get my mod working.
First issue to resolve is to make sure CURL is configured on your hosting provider. To verify contact your host and ask. I was hosted on a windows machine using webhost4life and caused some issues but working with them I was able to resolve it. I also used the "sample php" code on the http://developer.authorize.net/samplecode/ site and put in my user id and API key and was able to rule that issue out. It's generic code that will allow you to run a test and return the error codes from authorize to tell if you are getting values back. If you get a success with that code CURL and your API login credentials are most likely setup correctly.
A couple of things to remember with these modules:
1. If you have a "test" account set up you will want to use the test mode but if you have a real authorize.net account setup with it in "test mode" you will want to run everything in "live" settings mode in the module.
2. Set Transaction version = 3.1 (this is AIM) in the Authorize.net settings
3. Set your Direct Response Delimiter in the Authorize.net settings
Delimited Response: no
Field separator: comma(,)
Field Encapsulation Character: blank
4. Set your response/receipt URLs in your authorize.net settings
https://yourdomian.com/catalog/checkout_confirmation.php (Default Receipt URL)
https://yourdomain.com/catalog/checkout_process.php (Default Relay Response URL)
5. Be careful using these but use a logger on these modules - in my case I was making it to through the OSC shopping cart process only to find that I was not passing the CC info to authorize because of an incorrect </form> tag in the checkout_confirmation.php code. I ended up using the authorizenet_cc_aim.php module as it was the most updated at the time and found this post to be spot on to the issue I was having of not passing the CC data and getting the error sending me back to the payment page - http://forums.oscommerce.com/index.php?sho...t&p=1280555. The post by Jeff Wigal also includes a logger for viewing the info you are passing to authorize and the response you are getting back from them. Again make sure you delete the logged info when you are done testing and delete or comment out the code. There is also a logger for the authorizenet_aim.php by Joshua Morris for the Authorize Net AIM module (GPL) - vger module here http://forums.oscommerce.com/index.php?sho...t&p=1283835
For me changing the checkout_confirmation.php tags worked. I did read through all these posts and followed the advice for setting up the MD5 hash as well. I am now able to run a credit card successfully so I am just trying to pass along what worked for me to others that maybe having the same issue. I am not a developer but these posts were easy enough to follow and test. I would recommend you backup any code that you will be changing and follow the advice of the posters about deleting any files that the loggers create - it's sound advice.