O manual do usuário do VirtualBox indica que a virtualização de hardware deve estar habilitada para que o SMP funcione adequadamente.
Even though VirtualBox does not always require hardware virtualization, enabling it is required in
the following scenarios:
VirtualBox’s 64-bit guest support (added with version 2.0) and multiprocessing (SMP,
added with version 3.0) both require hardware virtualization to be enabled. (This is not
much of a limitation since the vast majority of today’s 64-bit and multicore CPUs ship with
hardware virtualization anyway; the exceptions to this rule are e.g. older Intel Celeron and
AMD Opteron CPUs.)
Talvez a sua BIOS ofereça suporte à virtualização de hardware, mas a sua CPU não?