Did you test the process via commandline?
If so, there are chances that you might have missed some steps in between.
The correct process as far as I can remember would be:
create Aggr->volume->LUN
make sure that LUN is online
create an igroup, add ESXi's iqn to this i group
map the LUN to this igroup
configure vmkernel port for iscsi traffic on ESX
add target portal to vmware iscsi initiator (!IMPORTANT! - DO NOT use ip address od e0M port on your storage system)
and rescan..!
You could avoid a lot of configuration mistakes by using the OnCommand Systemmanager GUI to configure volumes and LUNs on yor netApp storage systems. Its free, and easy to use.
Available here: http://mysupport.netapp.com/NOW/cgi-bin/software/?product=OnCommand+System+Manager&platform=Windows
There is a Linux version also available, in case if you are not a fan of microsoft. http://mysupport.netapp.com/NOW/cgi-bin/software/?product=OnCommand+System+Manager&platform=Linux
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