From memory, you need a 64bit cpu, and need to enable hardware visualization support in the bios
I just found this on the virtual box Q&As
Q: Does VB support 64 bit guests?
A: Support for 64 bit guests was introduced at version 2.0.0. If you wish to create 64 bit guests then we recommend that you upgrade to the latest version. Note that VirtualBox also supports 64-bit guest operating systems, under the following conditions:
You need a 64-bit processor with hardware virtualization support (see User Manual chapter 1.2, Software vs. hardware virtualization (VT-x and AMD-V), page 10) and a 64-bit host operating system.
NOTE: Since version 2.1.0, you can run 64 bit Guests while running a 32 bit Host. The prior requirement for Operating System is only for version 2.0.x. You still need the proper hardware.
You must enable hardware virtualization through your BIOS; software virtualization is not supported for 64-bit VMs.
Note: some HP BIOSES have a glitch, When You ENABLE VT-X/AMD-V, You DISABLE it, so do try both ways.