This should be trivial, and it is with the right instructions. Unfortunately, those are not listed in the disclaimer given when connecting to the NIH FTP site. The NIH gives the instructions here. The NIH FTP offers a standard anonymous connection. Anonymous connections use “anonymous” as the user and the connecting user’s email as the password. Briefly, follow the command in a Unix terminal:
ftp ftp.ncbi.nih.gov
You should receive a prompt that looks like “Name (ftp.ncbi.nih.gov:dfiler):” where “dfiler” is your username. Enter “anonymous” into the prompt. Then, you get a message asking for your email to serve as a password. Enter a valid email and you have access to the site. You can then use standard FTP commands to navigate and download the files you need.