It seems your system and nic are not listed in the Compatibility Guide as a ESXi 4 supported hardware:
http://www.vmware.com/resources/compatibility/search.php?action=search&deviceCategory=io&productId=1&advancedORbasic=advanced&maxDisplayRows=50&key=82578DM&release[]=-1&datePosted=-1&partnerId[]=-1&manufacturer[]=-1&vid=&did=&svid=&ssid=&rorre=0
The ESXi 4 has not a driver embeded for your NIC.