You use that product key if your system came with a Trial version of Office, as you can see in the screenshot WR2 posted, it states "for 1 PRELOADED PC only" meaning it was already installed for you. If you have a clean Windows setup you need to buy the disk, or maybe download the trial version then buy the key.
Thanks for your time my friend. I'm happy to report that I actually purchased the Installation disk after all. Have a good Holiday.