I am getting the same error as this guy was. No response from DHL. However, I took a look at the Server Info tool in osCommerce and found that cURL and SSL are enabled:
curl
CURL support enabled
CURL Information libcurl/7.10.6 OpenSSL/0.9.7a ipv6 zlib/1.1.4
I e-mailed DHL XML support, and they ran a request using the same credentials I am using, and everything worked fine. I have tried to install this module on two separate installs of osCommerce on my server (one install was heavily modified, the other was the default MS2 install). Both of them get No reponse for the XML request. What other steps can I take to troubleshoot this issue?