The new feature can be found in the Home window. There you can connect to your Dropbox account or create a new (free) account using the feature Add Account. The Dropbox account will then become visible in the Storage section of the Home window.
At the moment Dopbox access is only available in the most recent desktop versions of Acrobat DC. In the upcoming months an iOS version for iPad and iPhone will follow. Next year Dropbox access will also be possible with Android and web view.