According to the HCL, the driver is included with ESXi 5.1 (GA), but missing/async with ESXi 5.1 Update 1. What you can do is either trying to install ESXi 5.1 (without Update 1) or building your own installation image with the appropriate driver injected.
You can download the driver from http://vibsdepot.hp.com/hpq/apr2013/esxi-5x-devicedrivers/ (I think it's the BCM-NetXtremeII-4.0-862948.zip), then use the ESXi-Customizer.
André
