Save the article PDF to your desktop or a folder on your computer.
From the desktop: highlight the relevant EndNote reference and drag and drop the PDF icon into the reference (this is easiest done with the desktop on show and your EndNote library window minimised)
From a folder: highlight the relevant EndNote reference, and while in your EndNote library click References>File Attachments>Attach File. Browse your folders for the PDF and then double click to attach it to the reference.