Have you looked at the Getting started with Reactional Music guide first?
It is important that you place it in the StreamingAssets > Reactional folder.
Either via the pop up dialog that will show when opening scenes that do not have it, or through the top menu.
If no Default Section has been set, the manager will automatically pick the first defined in the downloaded bundle.
Toggling Autoplay on either Theme or Track will start music playback on game start.