Here are a few suggestions:

Cybersource.php file
Make sure file “cybersource.php” is located in the root directory of your ZC store on the host server.

Receipt Response URL
Verify that the “Receipt response URL” used by CyberSource to return transaction results is properly specified (Paragraph 7 of Guidelines);

Go to CyberSource Business Center >> Test Business Center >> enter Username & Password and click Login >> Left vertical bar menu >> Tools & Settings >> Hosted Order Page >> Settings;

Under “Appearance >> Receipt Page”, in the “Receipt Response URL” input box type: https://yoursite/yourstore/cybersource.php?result=true
Replacing “yoursite” and “yourstore” with proper URL info for your ZC store. If your store is located in the root directory, the “yourstore” field is not present;
Check the following “This URL is my custom receipt page.” checkbox;
Leave all other fields untouched or blank.

Please also verify the correctness of the “Decline Response URL” further down the page.

cs Module in ZC Admin
Please review settings in “ZC Admin >> Modules >> Payment” for the cs module:
“Transaction mode” set to “test”, etc.

Validation messages from CyberSource response page
Please provide a list of erroneous API fields and corresponding messages returned by the CyberSource server. (you should have a “validation” column on that page).

deBeaujeu