Nesting ESXI 5.1 on a ESX 4.1 host

Hi guys,

One the nested ESXI 5.1, I can not start up a 64 bits O/S.

Error: 

This virtual machine is configured for 64-bit guest operating systems. However, 64-bit operation is not possible.

This host does not support VT.

For more detailed information, see http://vmware.com/info?id=152.

I am using a Physical HP Proliant DL380 G7 with ESX 4.1.

How do I enable VT on the ESXI 5.1's BIOS?

Thanks,

Tom