It won't work under gift cards and the author has never sent out a Nook claim code before so she doesn't know what to do.
I tried chatting with Nook Customer Service and they said to ask the person who gave you the code.
Might this be the same procedure when you purchase a NOOK book in store and there is a redemption code on the receipt?
I have never done this, but perhaps someone who is familiar with the procedure can chime in here.
If you have a Galaxy Nook or a Glowlight you can also redeem access codes in Shop settings.