Video by PG Music Inc. via YouTube
Source
This video shows how to search for a Vocal Oohs RealTrack for your Band-in-a-Box song, as well as find a specific RealTrack. The steps are shown in Band-in-a-Box for Windows, but they are the same steps in the Mac version. To open the RealTracks Picker on Mac, click on the track in the Mixer, then the menu items are labelled as Select RealTracks | Choose RealTracks.
This video also shows the general steps for adding RealTracks to your song: https://youtu.be/nbf9P496sy0
This video goes over selecting the best RealTracks for your song: https://youtu.be/M5hgQOfiz74