Let me start by saying that this is a bad idea: running an ESXi host in VMware Workstation on a VM running within Fusion. You're getting into I N C E P T I O N territory here.
The intrepid among you may forge ahead regardless of the warning. If so, here's a quick tip.
When you first created your VM in Fusion, you probably skipped over an Advanced Option in the Processors & Memory page. If so, and you try to boot ESXi in a Workstation VM within your Fusion VM, here's what you'll see -->
In Fusion, go to your VM's settings page, and click Processors & Memory. Here's what you're looking for:
Why would you want to do this? Have you heard of CloudCred? :)