There is no product key for NSU's EndNote installation file. You download a compressed "zip" folder containing several files. You must save this folder to your computer, then extract or "unzip" the folder. The folder icons for a compressed vs extracted will look different -- one will have a "zipper" on it, and the other won't.
You have to click the installation file from the extracted/unzipped folder.
If you try to install the software via the compressed folder the software will ask you for a product key and/or install as a trial version -- this is a good sign that you have clicked into the wrong folder.