You don't want to conditionally format the check box (which
is pretty useless anyway), you want to format something else
depending on the value in the check box.

You can not conditionally format the form's (Report?) detail
section, but you can add a large text box that's sized and
positioned to filee the detail section.  Set its Conditional
Formatting - Expression Is option to [thecheckbox] = True
and then use Format - Send to Back to move the text box
bhind all the other controls.  Youwill probably also want to
set some of the other controls BackStyle property to
Transparent.