It sounds like the HAL for your VM is set to a uniprocessor configuration. You can "upgrade" it to a multiprocessor HAL fairly easily though. Download the devcon utility, and place the extracted executable in the same directory as this script (save it as mphal-upgrade.bat):
@title "Upgrading to ACPI Multi-Processor HAL.."
echo Upgrading to ACPI Multi-Processor HAL..
echo please wait..
devcon sethwid @ROOT\PCI_HAL\0000 := !E_ISA_UP !ACPIPIC_UP !ACPIAPIC_UP !ACPIAPIC_MP !MPS_UP !MPS_MP !SGI_MPS_MP !SYSPRO_MP !SGI_MPS_MP > nul
devcon sethwid @ROOT\ACPI_HAL\0000 := !E_ISA_UP !ACPIPIC_UP !ACPIAPIC_UP !ACPIAPIC_MP !MPS_UP !MPS_MP !SGI_MPS_MP !SYSPRO_MP !SGI_MPS_MP > nul
devcon sethwid @ROOT\PCI_HAL\0000 := +ACPIAPIC_MP > nul
devcon sethwid @ROOT\ACPI_HAL\0000 := +ACPIAPIC_MP > nul
devcon update %windir%\inf\hal.inf ACPIAPIC_MP > nul
echo Script Completed: press any key to reboot..
pause > nul
After rebooting you should be able to open the Device Manager, expand the Computer entry and see that the ACPI Multiprocessor HAL device is loaded correctly. The Task Manager should also show that you now have two CPUs available.
Credit for script: http://www.pimp-my-rig.com/2008/08/article-acpi-uniprocessor-to.html