PassThrough involves not only Virtualization to be configured, but IOMMU (VT-d). I have a DL380 G5 and it is not IOMMU (PassThrough) compatible, so yours probably isn't either. According to your server model spec sheet, I am not seeing VT-d in the processor specs:
http://h18004.www1.hp.com/products/quickspecs/13012_na/13012_na.pdfhttp://h10010.www1.hp.com/wwpc/us/en/sm/WF06a/15351-15351-3328412-241644-241475-3646113.htmlYou get told if PassThrough is capable on your host within the vSphere Client -> Config tab -> Adv Features.
~coolsport00