If you are adding VR to your Elite: Dangerous experience, you've already purchased the game. This means you need to share this information with either Oculus Home or SteamVR so these stores do not try to charge you again for the game.
Head to the Frontier website in your browser, log in to your account, and go to your Account Dashboard. Here you will see an option labeled Partner Keys, which will allow you to claim a key for Elite: Dangerous on your VR platform of choice. Redeem this code on your VR store, and you'll be able to download the VR version of the game from those stores.
This does mean you'll need to uninstall the version of the game you currently have installed. There is no way to enable VR mode for Oculus Rift or SteamVR from the standard PC version of the game anymore. Before you uninstall the game, be sure to head to Users/[accountname]/AppData/Local/Frontier Developments/Elite Dangerous/Options/Bindings/ in Windows and move the "custom.binds" file until the game is re-installed. This will save you from needing to re-map your keys in the new version of the game, just in case that information gets lost.