We do something similar using a JavaScript calendar.
First our form has a name="myform" (or something similar) tag.
Next you have to have the field named in the form, we call ours date. Then we have a button that activates the pop-up JS for the calendar.
This is where it gets tricky, you have to specify the name of the form field that the calendar assigns the value to. AND you have to specify the name of the form that you are using - document.formname.fieldname
Here's a sample of our code:
<form method="post" action="setdate.cfm" name="myform">
<input type="text" name="date">
<input name="cal" type="button" value="Calendar" onClick="setDateField(document.myform.date); top.newWin = window.open('calendar.html')">
</form>
I can email you the JS and Html if you would like. Its free code we found on the web about a year ago.
Hope this helps!
Roger