Why add memory to your nook? If you want to store a large number of books beyond the internal memory you have a lot more books than I do!  Many people want to store pictures, audio books, and music on the nook, and the best place for those is on an additional memory card.  The speed of the card you purchase matters!  I'd recommend a class 6 or 8 Micro SDHC card. Amazon has some great deals on memory cards so check out the following deals if you haven't purchased yet.
Check Out some Memory Card Deals

Let's get started.

Remove Back Case & Position Card Holder

Turn OFF your nook by holding the off button for several seconds. Carefully remove the back cover (pull the clip up at the bottom of the nook and slowly pry it off). I'd read the instructions on page twenty-five from the manual if you haven't done it yet!

Locate the words, “Micro SD” on the page of your nook. There is a small metal holder that slides up and down by pushing it with your finger. Push it up toward the words “Micro SD”.  You should feel it click into place.

After you push it up it should look like this:

Pull up Card Case

Place your fingernail (do not use a metal device) under the bottom of the metal cage and pop or pivot it up.

After you have done this correctly it should look like this:

Insert the Card, Close and Lock Lid

The card should fit neatly into the holder with the straight side of the memory card on the left. The metal connectors on the card should be face down.

After inserting the card make sure you push the metal housing back down until it clicks into place.

You're Done!

Put the case back on, starting at the bottom with the nook face down on a flat surface. I would follow the directions in the nook guide to be sure. I found it simple, but don't press hard on it, especially in the middle.

Now when you plug-in your nook an extra device will appear for you to store content on!