About form builder and workflow parameter

I have some problems about the form builder and workflow parameter:

1. From the tutorial in BeanShell Plugin, can you explain clearer about how to set value to a workflow variable? I can't really understand the example.

2. For the form variable, can it be used to stored and display a single value at the form? Or it can only be used to display list for dropdown list, checkboxes and radio buttons?

3. I wish to have a simple checking on the form that, if I checked on a checkbox, the date picker will disappear or disable. But the checking for the checkbox just can't work. Below is the javascript:

<script type="text/javascript" src="http://localhost:8080/wflow-wfweb/js/jquery/jquery-1.2.6.pack.js"></script>
<script type="text/javascript" src="http://localhost:8080/wflow-wfweb/js/json/util.js"></script>
<script>
  $(document).ready(function(){
    var oneway = document.getElementsByName('one_way')[0];
    var returndate = document.getElementsByName('return_date')[0];
    $(oneway).change(function() {
      if(oneway.checked==true)
        returndate.style.visibility ="hidden";
    });
  })
</script>

   I have checked that the change() function is working, so the problem should be on the if statement. Do you know what is the cause?

4. As I try to hide or disable the date picker, only the text box will be hidden or disabled, but the label and the date picker link is still there. Is it anyway to hidden or disable the whole date picker?

5. Is it any place that I can check for the debug or error log that I print out in Beanshell plugin for easier debuging process?