If you are taking advantage of the free upgrade to Windows 8.1 it must be download and installed from the Store. If I understand you correctly you downloaded or are trying to download from a Microsoft Web site. That download is for those that have to purchase Windows 8.1. Hope this helps and if you need further assistance post and we will be glad to assist you.
The above opinion is mine and mine alone and doesn't necessarily reflect that of Microsoft, it's employees, or any other member of this forum.
"When we try to pick out anything by itself, we find it hitched to everything else in the Universe." -John Muir
Edit: No key needed when you download from the store