According to http://h18004.www1.hp.com/products/servers/vmware/supportmatrix/hpvmware.html the ML350e Gen8 is supported with ESXi 5.0 U1/U2 and ESXi 5.1, but you need to use the HP Customized ISOs for installation.
Have you tried these or were you using the vanilla VMware ISO?
You can find download links for the HP customized ISOs and other useful HP stuff on my HP & VMware Links page at http://www.v-front.de/p/hp-vmware-links.html.
- Andreas