While Nook tablets like the Nook Color, Nook HD, or Nook HD+ are treated like an Android tablet (see instructions here), downloading to a Nook Simple Touch or other dedicated e-reader (like the Kobo, some Sony e-readers, as well as others) is done a bit differently.
Downloading to a dedicated e-reader requires that you start the process of searching for and checking out an ebook by visiting one of our ebook platforms on your computer. You will also need to download the free Adobe Digital Editions (ADE) software to your PC or Mac. Using our Overdrive collection as an example, once you check out an ebook, hit "Download," and it will open in your new ADE software. The first time you use your software you will be required to register with a free Adobe ID. If you do not have an Adobe ID, you will be prompted to create one online. Then you plug your Nook or other dedicated e-reader in to your computer with the USB cable. You will be prompted to authorize your device with ADE the first time you plug it in. Then you can drag and drop your new ebook in to your e-reader in the ADE software.
Here's a video that walks you through the process.