To locate your game's activation/serial key, navigate to the top right corner of the Direct2Drive site and log into your account. (Note: I this is your first time logging in from the old D2D or Gamefly, you'll be prompted to update your password)
Once you're logged in, please proceed to your account, by clicking "My Account" in the top right-hand corner by your name.
Next, proceed to your "My Games" tab. Your game Key will be located to the right of your box art under "Get Product Code".
(!!) For games that do not use Publisher activation, 3rd-party (Steam, Origin, UPlay, etc) and are not MMO games, please have a look at this FAQ: How to Reactivate your GameFly-activated Game on Direct2Drive