I've been told the follow from a window user:
Hi Sparrow,

I have BIAB for Windows. How I enter lyrics is as follows.
Have a melody notated to which I want to attach lyrics. I use the Melody track for this.
Switch to Notation Mode
Click at the beginning of the bar where I want to start adding lyrics.
Use cntrl+L to begin entering lyrics at the current bar. (This can also be accessed via the Lyrics menu as Enter lyrics at current bar.)
The current note is highlighted red.
Type lyric in the lyric entry box directly above the staff in the middle of the screen. (cntrl+L toggles this lyric box on and off)
Hit enter to move on to the next note.
NOTE: If you make a mistake, don't worry that it seems difficult to get back to the note where the mistake is located. Simply click on the bar concerned and choose Edit lyrics as event list from the Lyrics menu. Locate the faulty lyric, double click on it and make the correction.

I'm not sure if this will work in the Mac version but it might give you some ideas to try.

very clear ! there isn't any doubts!
but being IMac user I tried to type in my lyrics on the stave with this help ... I don't think the difference from Mac and Window are so big ... it seems in Imac the way of work is more simply than window

but ... I didn't get anything
compared with window there isn't any correspondence with the notes and lyric, Biab open a window to type in the lyric that haven't any correspondence with notes, the notes aren't highlighted in red ( or highlighted at all) meanwhile I'm typing lyric,
so I'm wondering what is my mistake?
or in alternative
is there in Biab something that doesn't work correctly?
any people may help me?