"esxcli swiscsi nic add"currently has an issue where all errors result in the same message: "Failed to add nic."
You get this error when you specify an invalid vmk device or (more importantly) when the Port Group for the VMKernel device has more than one uplink available. There needs to be one vmnic as an "Active Adapter"; all other adapters must be listed as "Unused". You can not have an adapter in "Standby"or you will get this message.
VMware does have a KB article on this: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1009450