How to make 64-bit guests available in a nested VirtualBox installation on iMac 5K 27-inch?
At the present time VirtualBox does not officially support nested virtualization, even thought one can nest a 32-bit Guest a 64-bit guest cannot be nested as VT-x is not passed through to the nested Guest and it's one of the requirements to run a 64-bit Guest, nested or not.

VMware Fusion does support some nested 64-bit Guests although off the top of my head I'd have to check the documentation to be more specific.  You can have a look Running Nested VMs at VMware's website.