I have a 64 bit Windows 7 guest box and when trying to run a VirtualBox 64 bit CentOS 6 box within it, I am unable to get it to start, and receive the error above: "VT-x/AMD-V hardware acceleration is not available on your system" Mac OS 10.7.5 Parallels Build 8.0.18608
The full error: "VT-x/AMD-V hardware acceleration is not available on your system. Your 64-bit guest will fail to detect a 64-bit CPU and will not be able to boot."