A couple of things to try.. First, verify that the ISO is not corrupt by verifying the MD5 checksum. If things look fine there, reformat the USB stick or try a new USB stick and then try reinstalling ESXi.
Also, did a quick look on Zotac's website and didn't find anything that would indicate the device would support ESXi.
The ZOTAC is not supported by vmware and is missing from the compatibility checker. The only thing that might be worth trying is to create your own esxi installer disk with the drivers you require embedded.