ESXi is a fantastic piece of kit, allows you to do all sorts! As such, me, and others like me have a tendency to try and get it to run on everything from a Cisco UCS to the kitchen toaster.
Problem is, ESXi isn't very good at supporting the hardware in toasters so you need some VIB (VMware Information Base) files (essentially drivers) to accept hardware. A common one of these is for storage devices and NICs. Both of these are fairly essential both for running and even installing ESXi.
A rather good source of community driven VIB files can be found here:
These will often need installing into the ISO file from VMware in order to actually work. There are a few tools around for this, one of them is on that page. Another can be found here:
Note: This one only seems to work with windows 7, but it's a little faster to use (since you don't have to install powerCLI). It also implies it only works for ESXi 5.x however it seems to work without issue for 6.0 U2 as well so can't complain!