The following steps are documented from Barnes and Noble:
1) Connect your NOOK to your computer with your USB cable.
2) When your computer recognizes your NOOK, a NOOK drive will appear in your "My Computer" folder (PC users) or on your desktop (Mac users). Please note: For MAC OS users, when you connect your NOOK to your computer for the first time, you will see a NOOK drive come up with a file on the drive called "MyNOOK Setup." Double click on the file to prompt the setup and follow the on screen instructions. This is a one-time installation for in order to transfer files onto your NOOK using your computer running Mac OS.
3) To add eBook PDF documents, simply drag and drop or copy and paste the files into your NOOK drive's "my documents" folder. (Please note large files may not upload and format properly on your NOOK. If this is the case, please consult the next FAQ.)
4) When you want to view or select the documents you added, click on "My Library," tap "View My Documents," and tap "Check for new content." Choose one of the documents you uploaded and click select.
5) View a helpful video from Barnes and Noble on uploading PDF and ePub documents and other files
here.