MegaSign field merge

I'm on an esign 15 day trial and I'm trying to get a megasign working that automatically enters different info for each recipient. If I use their sample csv and only select their "Signer Info Fields" - "Signer Name" it will correctly include the person's full name. However if I try to include it on each agreement as a custom field using {{_es_signer_fullname}} I can't get any fields included in the sample csv or any of my custom fields that I want to include. Can anyone point out what I'm doing wrong as I'm sure it's something simple.

Thanks!