TOPIC: [SOLVED] What is correct response url for Authorize.net?

When using the AIM method (where the whole transaction takes place within your site), you do not need to set anything for the response URL in your account. However, when using the SIM method, to to the account settings for your Authorize.net account and set your "Relay Response URL" as:
Of course, replace the above "YOURWEBSITE.com" with your actual domain name. jstratos had it right in his reply

Is this true for both versions of Joomla (1.5 and 2.5)?
Because when we use that in Joomal 1.5 using SIM we get these errors after submitting card information:

Warning: require_once(/home/storeuc/public_html/components/com_dtregister/lib/payment/.php) [function.require-once]: failed to open stream: No such file or directory in /home/storeuc/public_html/components/com_dtregister/controllers/payment.php on line 241

Fatal error: require_once() [function.require]: Failed opening required '/home/storeuc/public_html/components/com_dtregister/lib/payment/.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/storeuc/public_html/components/com_dtregister/controllers/payment.php on line 241

The payment does go through but this error message is a problem for the users.

When you get the errors you have referenced here, it is indicating that the success.php file is not able to be accessed. This would most likely be a permissions issue or a restriction coming from your .htaccess file. For permissions, all directories (from the top level containing the Joomla install on down) should be 755 and files 644. Make sure especially that this file is accessible:

/components/com_dtregister/success.php

If you have special security restrictions in your .htaccess, this could cause a problem too. To test this, you can always temporarily load the default .htaccess file to your site and test the transactions. If it now works, you know you need to make changes in your modified .htaccess file.

Also, note that if your site is set in configuration as offline, you will likely have these type of issues also. Just stating that for those that could run into this problem and their site is offline during the period of development.

The file was set to 0644 as are all files. So, I changed permissions for this to 0755 if that is what you are implying here.

No, files should be 644. It is directories that should be 755. Not just that folder, but all the way up to the folder containing your Joomla install. If permissions are correct, then the file is not accessible due to something else, like a restriction in the .htaccess file.