on 02-23-2013 10:22 PM
You are prompted to do it when you turn on the nook for the first time. If you skip past that, you can do it online as someone else posted, or you can go into the settings area on the nook itself and under the "shop" area, you can manage credit cards and gift cards.
on 02-26-2013 08:29 PM
Actually, you can do either of the above.
The N1E required you to set up an account on a computer, which was annoying and frustrating to everyone.
The NC, NT, NST, NSTG, and HD Nooks all can create accounts from the device.
You'll be prompted to either login with an existing account, or to create an account if you do not have one already. Going through account creation will ask you to enter a CC as the default payment method. If you have an account already but that account lacks a default CC you'll need to add one by going into settings > shop (settings > application settings > shop on the HD) and selecting 'manage credit card'.