Once you have downloaded your product, unzip the folder and store it where you keep your libraries. Load up the required version of Kontakt and click the 'Files' tab which is located on the top left side of Kontakt's interface. From here you can navigate to where you have stored your library. You can either double click, or drag the NKI file into Kontakt. You can also drag an NKI directly from a folder on your computer if this is easier.