Virtualized Intel VT-x/EPT is disabled for this ESX VM. You will only be able to run 32-bit nested VMs

I am trying to install ESXi v5 in VMware Workstation v8. When the virtual machine is booting, I get a warning message saying:

Virtualized Intel VT-x/EPT is disabled for this ESX VM. You will only be able to run 32-bit nested VMs.

This is my computer’s processor:
Intel(R) Core(TM) i7 CPU
950  @ 3.07GHz, 3068 Mhz, 4 Core(s), 8 Logical Processor(s)

What must I do to make the warning go away?

Thanks