First off, have some patience. It took me 20+ minutes to get it set up initially, though they claim it may be up to 30 minutes.
This is all in your guide, by the way, but I'll give you a quick overview. You can create a MotoCast account (MotoCast ID) on the computer (after installing the Motorola software) or on the phone (under My Accounts). Connect your phone to the computer via microUSB cable. Install the Motorola Desktop software (and updates). Follow the on-screen directions to select which folders you want to include in MotoCast.
I'm not sure what kind of indexing it does, but the initial set up, as I said before, takes some time.
You can access the files through each appropriate program: Gallery shows pictures, Music Player streams music, etc. You can also open the "Files" app and look at all of them together in a Windows Explorer type of order.