Re: Win 8 Ubuntu 12 "kernel requires an x86-64 CPU, but only detected an i686 CPU"
In the VM settings there are special options you need to turn on to expose 64-bit mode to the guest OS. One is Nested Paging, the other is right above it but I can't remember its name.
Have a look and enable them and it will work. If you can't enable them, you are stuck with a 32-bit guest OS.
I try to treat the cause, not the symptom. I avoid the terminal in instructions, unless it's easier or necessary. My instructions will work within the Ubuntu system, instead of breaking or subverting it. Those are the three guarantees to the helpee.