Welcome to the Community,
make sure that while creating a custom VM that you select ESXi 5 as the guest OS. This will configure virtual "E1000" network adapters which should be detected by the default ESXi image.
If you want you can also change the current VM's settings by editing its configuration (.vmx) file and adding:
ethernetN.virtualDev = "e1000"
(where N is the number of the network adapter).
Make sure you make the chages with the VM's tab (or VMware Workstation) closed, so that the configuration file is re-read after editing it.
André