1. Are you using SIM or AIM? If SIM, try AIM.
2. Make sure you are using your API Login and transaction key. Some people get them confused (very common mistake) instead of your account login and password.
3. I don't see a path to cURL though it is probably not needed.
4. It is not quite clear to me if you have your account in test mode or not? I see your cart is in test mode. So make sure your account is in test mode as well.
5. Are you accessing the gateway through a secure site? If so make sure all of your secure settings are correct and your certificate is installed correctly and working. If you are accessing the gateway through a non-secure site make sure all of your secure settings are turned off.
6. Try to verify from your account that a transaction was attempted even though it failed. This can help you determine whether your cart/module is even communicating with your gateway.
7. Do a search in the osCommerce forums for your error. Maybe someone before you had the same issue and posted how he corrected it.