send email workflow for case entity

I just customize a new field in my case entity called "emailadd". when i create a new case, i will fill in the customer email address into this field "emailadd". So after i save the case, i will generate a workflow email and send to my customer. When i create the workflow , i try to attach this "emailadd", but i cannot find it from the list. See link below http://i1208.photobucket.com/albums/cc362/samuelccm/test_zps49f7662a.png

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