Welcome to the Community - When you say you "added vkernal using the UP of the NAS" I assume you are referring to a VMkernel Port with an IP address that is on the same subnet as the NAS device and not the same IP address as the NAS device because it needs to be a unique IP address.
Are you using the software iSCSI initiator included with ESXi? Check out page 82 of http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-511-storage-guide.pdf