All,
I had this same problem today but it turned out that since all my VM's were created in Vmware workstation 7, HPET wasn't enabled and was the reason for this. So, while most of the comments here provide some of the correct steps to take, dont think any of them include all the steps required (assuming that your nested hypervisor VM's were NOT freshly created in Workstation 8 with the HPET support turned on (if this is on, when you go to settings and select processors, on the bottom right, you'd see 2 check boxes and the one that says "Virtualise Intel VT-x/EPT or AMD-V/RVI" should be available.)
So, if you power the VM off, select it in the left hand pane within the workstation, you will see on the right to "Upgrade this virtual machine" select that, select the Hardware compatibility to Workstation 8 to enable HPET, select Alter this VM and when you click next, you'll see the changes to apply include Adding: HPET.
Once thats done, go to the settings of the VM (while powered off) and select the processors on the left and make sure the checkbox "Virtualise Intel VT-x/EPT or AMD-V/RVI" is checked. And also make sure the guest OS is selected as ESXi 5 under options.
Thats all you need to do.
if this is to be done through editing the vmx file. amongst the following is the key
hpet0.present = "TRUE"
If the VM was freshly created on Workstation 8, this would be there by default (provided that you used the ESXi5 installation CD during the creation time)
Hope this makes sense. (& dont forget to award points)
ChanEK