how to add/edit pitches in 3-line stave

Hi,

I have a 3-line stave but i can only add pitches by cutting/pasting from a 5-line stave. I can't enter pitches any other way, and it only allows notes on the top line of the 3, and then I can't use up/down to alter them (when I tried that it just removed the accidentals from the copy/pasted notes).

Obviously with a 3-line stave I'm not expecting 'proper' pitches, I just need to to be able to enter rhythmic noteheads on any of the three lines. What am I missing here...?

Thanks,
Scott