Excuse me for this rather silly question, but where do Ienter the code like B1G1 when I'm trying to fulfill an account? I don't see a box in the shopping cart, or during checkout.

It's toward the top of the main page after you log in. Click on "Looking for the offer in your mailing?" Don't feel bad. I am a CH newbie too, and it took a while to figure this out! :-)