Click on my computer and your nook and sd card should show up. click and drag the file where you want it to go. if your sd card doesn't show while installes in your nook. you will need to take it out and use a card adaptor to insert it into your computer. I had the same question and nook support helped me. I hope this helps you too.
I installed a new MicroSD card to my Nook Color. Although I've had my NC for over a year, this is the first time I've used a memory card in the NC.
How do I save books, etc to the card and not the device memory?
TIA!