You will need to install Adobe Digital Editions on your desktop computer (Mac or Windoze). Use it to download the books (the download link on Library site will start ADE. Next, plug in your nookColor and use ADE to "authorize" it (just first time). Then simply drag the book cover onto the nook (on left side of ADE). Eject the nook and the book will appear under "My Documents"!
Watch our Video Tutorial on using the nookColor with ADE before you get started if you are using content you are purchasing (there are some gotchas)