VirtualBox in Knoppix 7.0.5 can't run rhel_6.0 x86_64 iso

[rajibando]

Just as I have said, VirtualBox in Knoppix 7.0.5 can't run rhel_6.0 x86_64 iso. It stops with message:

This kernel requires an x86-64 CPU, but only detected an i686 CPU. Unable to boot - please use a kernel appropriate for your CPU.

Now, I have a x86-64 CPU as the hardinfo report states. But still, the problem in identification. Is there anything wrong with the kernel? The same message appears in Knoppix VirtualBox.
The same happens with Squeeze 6.0.3, and I have posted the same message with appropriate changes in the Debian Forum, with the subject, VirtualBox in Squeeze 6.0.3 can't run rhel_6.0 x86_64 iso

Regards