Installing 64-bit Ubuntu on VirtualBox fails with this error:
This kernel requires an x86-64 CPU, but only detected an i686 CPU. Unable to boot - please use a kernel appropriate for your CPU.
First, make sure you have a 64-bit CPU. Second, make sure it has support for virtualization. If both of these are true, then it means that some of the virtualization features might be turned off. Reboot your PC, get into the BIOS setup and turn on all the virtualization features of your CPU. 64-bit Ubuntu should install without this error after that.
Tried with: Ubuntu 11.10 64-bit and VirtualBox 4.1.12 on a Intel Core 2 Duo E6550 CPU