I think you're going to have to find a way to get the electronic book transferred to your account from your mom's account. Or perhaps B&N would take the book back from your mom as a return (electronically), and then transfer the credit to your account so you can purchase it on your account. If you're having trouble getting through to customer support on the phone, is there a B&N store nearby? You could try getting this taken care of in person.
I doubt that BN will transfer the book to another account. The easiest solution is to download the book to a computer and then sideload it to the NC. When you open the book it will ask for the Name and Creditcard info used to purchase it. Ask your Mom for this info, or ask her to enter the info and the book will open. She need not worry about you having her card info, the NC does not save the card number but it does save a code generated from the card number.
For future reference there is at present no way to purchase an ebook for someone else. The best you can do is get them a BN gift card which they can register to their account.
I think you should be able to download a copy to her computer, connect your Nook and sideload the book to the Nook's My Documents folder. Eject your Nook from the computer. When you try to open the book, you may be prompted for credit card info, enter hers and it should open.
Ed_Mart is too quick for me.
I received my Nook Color for Christmas and I absolutely love it, but I have encountered a very frustrating problem. My Mom purchased the Nook Color and a book through her B&N account for me for Christmas, however, I can't download the book onto my NC. We tried logging in to her account to download it to my NC but it has never downloaded. I have been working on this since Christmas and am getting very frustrated. Is this happening because my NC is registered to my account rather than hers? I have tried contacting customer support and have yet to get a hold of anyone, I also checked the boards to see if this has been asked before but I couldn't find anything, so I apologize if this same question has been asked frequently. I have also checked my user guide and checked google but can't find an answer.
You've already gotten responses from others, but for future reference, you may want to suggest to your family that they buy you B&N gift cards. That way you can register the gift card on your account and won't run into this problem.