Hi,
A couple of things:
I would put the script in the click event and rather than work with ".isNull", I would refer to the objects value (on = 1 and off = 0).
Have a look at script in the click event. I have wrapped all the components in a subform and the script shows and hides this in a single line. Also I have suggested a script to give uer feedback to the checkbox. One other thing you could consider is making the tickbox presence set to "visible screen only" so that it doesn't come out in the printed version.
Hope this helps.
N.