sk713,
While using that iso should work I would recommend for you HP server using the HP oem version of esxi5.1 . You can find this iso located here: http://h18004.www1.hp.com/products/servers/software/vmware/esxi-image.html . Using the OEM makes it easier most of the time because all the HP specific drivers will be preloaded in this version. Also, if your making a usb key of the iso I would recommend using rufus located here http://rufus.akeo.ie/ to build your usb key. Also, stating the obvious but you do need to make sure you select boot from usb or DVD when booting up your system.