Adding fields based on a check box.

I think this might be the last question from the javascrip newbie for a while. The last question is thi: I have some check boxes, and when they are checked I want 3 fields to be added together, otherwise let the result be either null or 0.
Check box: Skill
Fields: Mod, Rank, Misc
so if skill=checked then mod+rank+misc; else 0/null
 
Any help would be appreciated, I tried changing another formula:
 

var nSum = 0;
var aCkFlds = this.getField("MyChecks").getArray();
for(var i=0;i<aCkFlds.length;i++)
{ 
   if(aCkFlds[i].isBoxChecked(0))
      nSum++;
}
event.value = nSum;

 
thomp gave this formula to another user, but I just don't know enough javascript to be able to fix it, if a fix is what's needed rather than a complete different formula.
 
Thank you.