There is no product key for the NU version of EndNote, nor can you upgrade to the NU version from a trial version. If prompted for a product key, stop the installation, return to the zipped folder (likely in your Downloads folder) and follow the instructions below. If you have installed the trial version, you will need to uninstall it and then follow the instructions below:
When downloading EndNote from the NUIT website select "Save File". A zipped folder will be downloaded to your computer. Do not double click on the zipped folder. Instead, right click and extract the contents of the folder to a convenient file folder on your computer. Then use the Installer file inside the Extracted folder.