I use the lyrics document too but unless you manually enter the chords as well it's not much use in learning to sing the song. It's also redundant to have to do so since one presumably has already entered the chords in the chord display. The only thing that kind of sort of works is entering note based lyrics but it's time consuming and the lyrics don't always show up where they are supposed to in the lead sheet or when printed.

In case anyone is interested, it appears the karaoke window only opens up when you load a standalone audio file (with an associated CDG file) and not when you import audio into an existing BB song.