First off, I hope you didn't buy the EoF digital download hoping to get the original game plus all of the expansions that are out. That option is only available with the boxed version of EoF.
If you do have the original and purchased EoF digitally, the game will automatically detect that and begin downloading it the next time you log into the launch pad. If you purchased it from the launch pad, close the launch pad and restart the game. It will not say "Downloading EoF Now" or even give you an option to do so. Instead, it will look like the game is patching itself which, in essence, is exactly what it is doing. When you actually get in game, you will have all the benefits of EoF. Hope that helps.