I find that the product key (orange/yellow sticker inside the box) to be difficult to read. The font is not the best to discern sbtle differences in "I" and "J," for example. Try using a magnifying glass to confirm that you are typing the correct characters.
If you are still unable to activate, contact support. With a scan or a digital photo of the key, they will be able to verify the key for you.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve