One bar of 6/8 is equivalent to 2 beats of 4/4 in a triplet feel, so when you select 6/8 in the Notation Window, it displays each bar (as counted on the chord sheet) as two bars with two beats each, and six divisions in each beat that you can enter notes on.

The reason there are 6 lines per beat instead of 3 is that Band-in-a-Box's Notation Window always displays 16th note divisions, not 8ths. If you want to enter 8th notes, you just need to enter a note on every second line instead of every line.

I hope this helps!
Kent
PG Music