The "Execute Disavle Bit" setting can usually be found under "Security" in most BIOS versions.
To be able to run nested 64 bit VM's you may need to upgrade to VMware Workstation 8 which offers support to “Virtualize Intel VT-x/EPT or AMD-V/RVI” in the virtual machine's - the ESXi VM in your case - CPU options (with virtual hardware version 8).
André