If you get this message in Virtualbox :
Unable to boot - please use a kernel appropriate for your CPU
There are a few things you can do to make it work.
Reboot your physical machine and go in your BIOS at start.
Enable Intel VT (Virtualization Technology) or AMD-V (AMD Virtualization) depending on your processor's type.
If you don't have this option, try solution 2.
Run VirtualBox.exe, click Settings, select System in the left pane then go in Processor tab.
Check Enable PAE/NX.
This message could mean that you are trying to run a 64 Bits operating system with a non 64-Bits compatible processor.
Recreate your Live USB using a standard 32 Bits ISO instead of a 64 Bits one.