So you're running the PCI compliant version. I'm not much help in that department...sorry.
CRE managed to take open source software and tunnel you into using THEIR services for additional money. I worked with their new PCI version for about a month until I just said fuck it. They make it nearly impossible to not use their services for one thing or another.
Sorry about your luck and I wish I could help. You could maybe uninstall the authorize.net module you found and look for another version (make sure it's compatible with v6.4). You may have inadvertently installed a non-compatible version.
I wish I could help more but I dropped that poop bag after they went PCI compliant and tried to turn OSCommerce into the next Amazon.com software bundle.