You should create a MouseUp event on your "Select All" check-box, which test its value, and then iterates over the other boxes, setting their state. Something like this:
var otherCheckBoxes = ["CheckBox1", "CheckBox2", "CheckBox3"]; // etc.
if (getField("Select All").value!="Off") {
for (var i in otherCheckBoxes) getField(otherCheckBoxes[i]).checkThisBox(0,true);
}